iOS HW H264 support.

First step towards supporting H264 on iOS. More tuning/experimentation
required in future CLs. Tested using AppRTCDemo on iPhone6 + iPad Mini.
Future work to get it working on OS/X, simulator (renders black screen
currently) and with the Android AppRTCDemo. Currently protected with a
compile time guard.

BUG=4081
R=andrew@webrtc.org, haysc@webrtc.org, holmer@google.com, jiayl@webrtc.org, kjellander@webrtc.org, pbos@webrtc.org, phoglund@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1187573004.

Cr-Commit-Position: refs/heads/master@{#9515}
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index 3b54b56..1035b79 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -40,6 +40,9 @@
       "WEBRTC_IOS",
     ]
   }
+  if (is_ios && rtc_use_objc_h264) {
+    defines += [ "WEBRTC_OBJC_H264" ]
+  }
   if (is_linux) {
     defines += [ "WEBRTC_LINUX" ]
   }