Enable H.264 video WebRTC behind run-time flag and add WebRtcBrowserTest for H.264

chrome/chrome_tests.gypi

Index: chrome/chrome_tests.gypi
diff --git a/chrome/chrome_tests.gypi b/chrome/chrome_tests.gypi
index d09e44b4b3a1a4cc055e95a853720b14e61375de..2eb5f9c6110a157b21fb35f8b20518d898a00b86 100644
--- a/chrome/chrome_tests.gypi
+++ b/chrome/chrome_tests.gypi
@@ -2127,6 +2127,9 @@
       # Executable that runs each browser test in a new process.
       'target_name': 'browser_tests',
       'type': 'executable',
+      'includes': [
+        '../third_party/webrtc/build/common.gypi',
+      ],
       'dependencies': [
         'browser',
         'chrome_features.gyp:chrome_common_features',
@@ -2159,6 +2162,7 @@
         '../components/components.gyp:ui_zoom_test_support',
         '../components/components_resources.gyp:components_resources',
         '../components/components_strings.gyp:components_strings',
+        '../content/content.gyp:feature_h264_with_openh264_ffmpeg',
         '../content/content_shell_and_tests.gyp:content_browser_test_base',
         '../crypto/crypto.gyp:crypto_test_support',
         '../device/bluetooth/bluetooth.gyp:device_bluetooth_mocks',
@@ -2434,7 +2438,12 @@
             'browser/media/chrome_webrtc_browsertest.cc',
             'browser/media/chrome_webrtc_disable_encryption_flag_browsertest.cc',
             'browser/media/chrome_webrtc_getmediadevices_browsertest.cc',
-         ],
+          ],
+        }],
+        ['rtc_use_h264==1', {
+          'defines': [
+            'BUILDFLAG_RTC_USE_H264',
+          ],
         }],
         ['OS=="win"', {
           'sources': [