H.264: Default flags and pulling in openh264 and ffmpeg.

webrtc/build/webrtc.gni

Index: webrtc/build/webrtc.gni
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index 31f0ef501b23f36b5415e59b730a0cca9dfe1f97..c55f4230bdec9016d3c0d563fe775521906b7b3a 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -90,6 +90,17 @@ declare_args() {
   # Enable this to use HW H.264 encoder/decoder on iOS PeerConnections.
   # Enabling this may break interop with Android clients that support H264.
   rtc_use_objc_h264 = false
+
+  # Enable this to build H.264 encoder/decoder using third party libraries.
+  # Encoding uses OpenH264 and decoding uses FFmpeg. Because of this, OpenH264
+  # and FFmpeg have to be correctly enabled separately.
+  # - use_openh264=true is required for OpenH264 targets to be defined.
+  # - ffmpeg_branding="Chrome" is one way to support H.264 decoding in FFmpeg.
+  #   FFmpeg can be built with/without H.264 support, see 'ffmpeg_branding'.
+  #   Without it, it compiles but H264DecoderImpl fails to initialize.
+  # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
+  # http://www.openh264.org, https://www.ffmpeg.org/
+  use_third_party_h264 = false  # TODO(hbos): To be used in follow-up CL(s).
 }
 
 # Make it possible to provide custom locations for some libraries (move these