Reland of move {media,p2p,pc,xmllite,xmpp}_tests.gypi files. (patchset #1 id:1 of https://codereview.webrtc.org/1846693002/ )

The re-land moves the isolate build targets for media.gyp
and pc.gyp into the include_tests==1 condition.
This has been tested in a Chromium checkout and no longer
causes the error that was seen after landing
https://codereview.webrtc.org/1839763004/

Original issue's description:
> Revert of Remove {media,p2p,pc,xmllite,xmpp}_tests.gypi files. (patchset #1 id:1 of https://codereview.webrtc.org/1839763004/ )
>
> Reason for revert:
> Breaks Chromium: https://build.chromium.org/p/chromium.webrtc.fyi/builders/Mac%20Builder/builds/11313/steps/gclient%20runhooks/logs/stdio:
>
> Updating projects from gyp files...
> Using overrides found in /Users/chrome-bot/.gyp/include.gypi
> Traceback (most recent call last):
>   File "src/build/gyp_chromium", line 12, in <module>
>     execfile(__file__ + '.py')
>   File "src/build/gyp_chromium.py", line 341, in <module>
>     sys.exit(main())
>   File "src/build/gyp_chromium.py", line 328, in main
>     gyp_rc = gyp.main(args)
>   File "/b/build/slave/Mac_Builder/build/src/tools/gyp/pylib/gyp/__init__.py", line 538, in main
>     return gyp_main(args)
>   File "/b/build/slave/Mac_Builder/build/src/tools/gyp/pylib/gyp/__init__.py", line 514, in gyp_main
>     options.duplicate_basename_check)
>   File "/b/build/slave/Mac_Builder/build/src/tools/gyp/pylib/gyp/__init__.py", line 130, in Load
>     params['parallel'], params['root_targets'])
>   File "/b/build/slave/Mac_Builder/build/src/tools/gyp/pylib/gyp/input.py", line 2800, in Load
>     RemoveLinkDependenciesFromNoneTargets(targets)
>   File "/b/build/slave/Mac_Builder/build/src/tools/gyp/pylib/gyp/input.py", line 1510, in RemoveLinkDependenciesFromNoneTargets
>     if targets[t].get('variables', {}).get('link_dependency', 0):
> KeyError: '/b/build/slave/Mac_Builder/build/src/third_party/webrtc/media/media.gyp:rtc_media_unittests#target'
> Error: Command '/usr/bin/python src/build/gyp_chromium' returned non-zero exit status 1 in /b/build/slave/Mac_Builder/build
> Hook '/usr/bin/python src/build/gyp_chromium' took 20.29 secs
>
> Original issue's description:
> > Remove {media,p2p,pc,xmllite,xmpp}_tests.gypi files.
> >
> > These contributes to circular dependency problems in WebRTC
> > since one have to depend on webrtc.gyp in order to depend on
> > a target in them.
> >
> > This reduces the number of cyclic dependencies in WebRTC from 21
> > to 16.
> >
> > BUG=webrtc:4243
> > NOTRY=True
> > NOPRESUBMIT=True
> >
> > Committed: https://crrev.com/231b69f28dd22f4e2d98e5048f8eaae7b20915e6
> > Cr-Commit-Position: refs/heads/master@{#12166}
>
> TBR=pthatcher@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:4243
>
> Committed: https://crrev.com/72644d2cf6b14bbc4a107f79158eaa225f3196b5
> Cr-Commit-Position: refs/heads/master@{#12167}

TBR=pthatcher@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:4243

Review URL: https://codereview.webrtc.org/1843193002

Cr-Original-Commit-Position: refs/heads/master@{#12180}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 00984ff68891a18113261c09770650460c9de11b
diff --git a/webrtc_tests.gypi b/webrtc_tests.gypi
index cf72d2e..7840e80 100644
--- a/webrtc_tests.gypi
+++ b/webrtc_tests.gypi
@@ -18,9 +18,9 @@
         'libjingle/xmpp/xmpp.gyp:rtc_xmpp',
         'p2p/p2p.gyp:rtc_p2p',
         'p2p/p2p.gyp:libstunprober',
-        'rtc_p2p_unittest',
-        'rtc_xmllite_unittest',
-        'rtc_xmpp_unittest',
+        'p2p/p2p.gyp:rtc_p2p_unittest',
+        'libjingle/xmllite/xmllite.gyp:rtc_xmllite_unittest',
+        'libjingle/xmpp/xmpp.gyp:rtc_xmpp_unittest',
         '<(DEPTH)/testing/gtest.gyp:gtest',
         '<(DEPTH)/testing/gmock.gyp:gmock',
       ],
@@ -360,32 +360,6 @@
           ],
         },
         {
-          'target_name': 'rtc_media_unittests_run',
-          'type': 'none',
-          'dependencies': [
-            'rtc_media_unittests',
-          ],
-          'includes': [
-            'build/isolate.gypi',
-          ],
-          'sources': [
-            'rtc_media_unittests.isolate',
-          ],
-        },
-        {
-          'target_name': 'rtc_pc_unittests_run',
-          'type': 'none',
-          'dependencies': [
-            'rtc_pc_unittests',
-          ],
-          'includes': [
-            'build/isolate.gypi',
-          ],
-          'sources': [
-            'rtc_pc_unittests.isolate',
-          ],
-        },
-        {
           'target_name': 'video_engine_tests_run',
           'type': 'none',
           'dependencies': [