rtc_use_h264 flag for building OpenH264/FFmpeg

webrtc/build/common.gypi

Index: webrtc/build/common.gypi
diff --git a/webrtc/build/common.gypi b/webrtc/build/common.gypi
index 2c24963f4f3a9898e0a2953741121172ab2523b0..fea38db63574853394172a4df13a22f07a85a583 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -135,18 +135,22 @@
     # Enabling this may break interop with Android clients that support H264.
     'use_objc_h264%': 0,
 
-    # 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=1 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.
+    # Enable this to build OpenH264 encoder/FFmpeg decoder. This is supported on
+    # all platforms except iOS. Because FFmpeg can be built with/without H.264
+    # support, |ffmpeg_branding| has to separately be set to a value that
+    # includes H.264, for example "Chrome". If FFmpeg is built without H.264,
+    # compilation succeeds but |H264DecoderImpl| fails to initialize.

[hbos, 2016/01/19 11:01:27]

(Not asserting that ffmpeg_branding has to specifically be "Chrome", might be
other valid configurations)

     # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
     # http://www.openh264.org, https://www.ffmpeg.org/
-    'use_third_party_h264%': 0,  # TODO(hbos): To be used in follow-up CL(s).
+#    'use_h264%': 0,  # TODO(hbos): enc/dec in follow up CL(s).
 
     'conditions': [
+      ['OS!="ios"', {
+        'use_h264%': 1,
+      }, {
+        'use_h264%': 0,
+      }],

[hbos, 2016/01/19 11:01:27]

Know where to do "override defaults" similar to build_overrides/webrtc.gni?
There's gyp_webrtc of course but I wasn't sure about the "if OS != ios" then and
I didn't find something similar to build_overrides/ so I placed the condition
here (temporarily, to be removed before landing)

[kjellander_webrtc, 2016/01/20 19:28:45]

I believe I've told you before, it's
https://chromium.googlesource.com/external/webrtc/+/master/webrtc/supplement....

[hbos, 2016/01/21 10:00:01]

Thanks. Yeah I believe you did, but I couldn't find it afterwards.

+
       ['build_with_chromium==1', {
         # Exclude pulse audio on Chromium since its prerequisites don't require
         # pulse audio.