Reland of GN: Enable ARC for Mac and iOS in rtc_* templates (patchset #1 id:1 of https://codereview.webrtc.org/2827223003/ )

Reason for revert:
Relanding after fixing ARC issue.

Original issue's description:
> Revert of GN: Enable ARC for Mac and iOS in rtc_* templates (patchset #3 id:40001 of https://codereview.webrtc.org/2781713004/ )
>
> Reason for revert:
> Breaks mac build
>
> Original issue's description:
> > GN: Enable ARC for Mac and iOS in rtc_* templates
> >
> > Remove all uses of retain/release and NSAutoreleasePool.
> >
> > This makes transformation to Bazel easier.
> >
> > This CL subsumes https://codereview.webrtc.org/2778163002 and depends on https://codereview.webrtc.org/2784483002/
> >
> > BUG=webrtc:6412
> >
> > Review-Url: https://codereview.webrtc.org/2781713004
> > Cr-Commit-Position: refs/heads/master@{#17780}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/6bda02b51dc00401d2a0d04be996ed10e5acac47
>
> TBR=kjellander@webrtc.org,magjed@webrtc.org,stefan@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:6412
>
> Review-Url: https://codereview.webrtc.org/2827223003
> Cr-Commit-Position: refs/heads/master@{#17784}
> Committed: https://chromium.googlesource.com/external/webrtc/+/7c8786ae8f2d8481f74b0ab16eb91a8101fe51dc

TBR=kjellander@webrtc.org,magjed@webrtc.org,stefan@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:6412

Review-Url: https://codereview.webrtc.org/2834273002
Cr-Commit-Position: refs/heads/master@{#17836}
diff --git a/webrtc/examples/BUILD.gn b/webrtc/examples/BUILD.gn
index 10deaf2..d8e2545 100644
--- a/webrtc/examples/BUILD.gn
+++ b/webrtc/examples/BUILD.gn
@@ -160,7 +160,6 @@
       "objc/AppRTCMobile/common/ARDUtilities.h",
       "objc/AppRTCMobile/common/ARDUtilities.m",
     ]
-    configs += [ "//build/config/compiler:enable_arc" ]
     public_configs = [ ":apprtc_common_config" ]
 
     deps = [
@@ -227,7 +226,6 @@
       "objc/AppRTCMobile/RTCSessionDescription+JSON.h",
       "objc/AppRTCMobile/RTCSessionDescription+JSON.m",
     ]
-    configs += [ "//build/config/compiler:enable_arc" ]
     public_configs = [ ":apprtc_signaling_config" ]
     deps = [
       ":apprtc_common",
@@ -260,7 +258,6 @@
         "objc/AppRTCMobile/ios/UIImage+ARDUtilities.h",
         "objc/AppRTCMobile/ios/UIImage+ARDUtilities.m",
       ]
-      configs += [ "//build/config/compiler:enable_arc" ]
 
       deps = [
         ":apprtc_common",
@@ -337,10 +334,7 @@
         "objc/AppRTCMobile/mac/APPRTCViewController.h",
         "objc/AppRTCMobile/mac/APPRTCViewController.m",
       ]
-      configs += [
-        "//webrtc:common_objc",
-        "//build/config/compiler:enable_arc",
-      ]
+      configs += [ "//webrtc:common_objc" ]
       deps = [
         ":apprtc_common",
         ":apprtc_signaling",
@@ -394,10 +388,7 @@
       "objc/AppRTCMobile/third_party/SocketRocket/SRWebSocket.h",
       "objc/AppRTCMobile/third_party/SocketRocket/SRWebSocket.m",
     ]
-    configs += [
-      "//build/config/compiler:enable_arc",
-      ":socketrocket_warning_config",
-    ]
+    configs += [ ":socketrocket_warning_config" ]
     public_configs = [ ":socketrocket_include_config" ]
 
     libs = [
@@ -430,7 +421,6 @@
           "//build/config/ios:xctest",
           "//third_party/ocmock",
         ]
-        configs += [ "//build/config/compiler:enable_arc" ]
       }
 
       rtc_ios_xctest_test("apprtcmobile_tests") {