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

webrtc/build/webrtc.gni

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
@@ skipping 72 matching lines @@
   }
 
   # Determines whether NEON code will be built.
   rtc_build_with_neon =
       (current_cpu == "arm" && (arm_use_neon || arm_optionally_use_neon)) ||
       current_cpu == "arm64"
 
   # 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
 # up into declare_args should we need to actually use them for the GN build).
 rtc_libvpx_dir = "//third_party/libvpx_new"
 rtc_libyuv_dir = "//third_party/libyuv"
 rtc_opus_dir = "//third_party/opus"