rtc_use_h264 flag (replacing use_third_party_h264 flag) for building OpenH264/FFmpeg, false by default but can be overridden in supplement.gypi and build_overrides/webrtc.gni.

BUG=468365
NOTRY=True

Review URL: https://codereview.webrtc.org/1601813005

Cr-Commit-Position: refs/heads/master@{#11333}

webrtc/modules/video_coding/codecs/h264/h264.gypi

@@ -23,6 +23,14 @@
             'h264_objc.mm',
           ],
         }],
+        ['rtc_use_h264==1', {
+          # Dependency for sake of compiling The dependencies will be used for
+          # real as soon as https://codereview.webrtc.org/1306813009/ lands.
+          'dependencies': [
+            '<(DEPTH)/third_party/ffmpeg/ffmpeg.gyp:ffmpeg',
+            '<(DEPTH)/third_party/openh264/openh264.gyp:openh264_encoder',
+          ],
+        }],
       ],
       'sources': [
         'h264.cc',