Enabling 'gn check' on webrtc/video.

I disabled the check on "video_tests" because it pulls
"//webrtc/media/rtc_unittest_main" as a dependency and it defines
the _main (that is already defined by "//webrtc/test:test_main").

I will file a bug to solve this in another CL.

BUG=webrtc:6828
NOTRY=True

Review-Url: https://codereview.webrtc.org/2832063003
Cr-Commit-Position: refs/heads/master@{#17859}
diff --git a/webrtc/pc/BUILD.gn b/webrtc/pc/BUILD.gn
index ff99c19..8e78caf 100644
--- a/webrtc/pc/BUILD.gn
+++ b/webrtc/pc/BUILD.gn
@@ -210,8 +210,9 @@
     deps = [
       ":libjingle_peerconnection",
       ":rtc_pc",
+      "../base:rtc_base_tests_main",
       "../base:rtc_base_tests_utils",
-      "../media:rtc_unittest_main",
+      "../media:rtc_media_tests_utils",
       "../system_wrappers:metrics_default",
     ]
 
@@ -356,8 +357,9 @@
       ":pc_test_utils",
       "..:webrtc_common",
       "../api:fakemetricsobserver",
+      "../base:rtc_base_tests_main",
       "../base:rtc_base_tests_utils",
-      "../media:rtc_unittest_main",
+      "../media:rtc_media_tests_utils",
       "../pc:rtc_pc",
       "../system_wrappers:metrics_default",
       "//testing/gmock",