Injectable Obj-C video codecs

Initial CL for this effort, with a working RTCVideoEncoder/Decoder for H264
(wrapping the VideoToolbox codec).

Some notes / things left to do:
  - There are some hard-coded references to codec types that are supported by
    webrtc::VideoCodec, cricket::VideoCodec, webrtc::CodecSpecificInfo etc
    since we need to convert to/from these types in ObjCVideoEncoder/Decoder.
    These types would need to be more codec agnostic to avoid this.
  - Most interfaces are borrowed from the design document for injectable
    codecs in Android. Some data in the corresponding C++ classes is discarded
    when converting to the Obj-C version, since it has fewer fields. I have not
    verified whether all data that we do keep is needed, or whether we might be
    losing anything useful in these conversions.
  - Implement the VideoToolbox codec code directly in the RTCVideoEncoderH264
    classes, instead of wrapping webrtc::H264VideoToolboxEncoder / decoder.
    Eliminates converting between ObjC/C++ types outside the ObjCVideoEncoder/
    Decoder wrapper classes.
  - List the injected codec factory's supported codecs in the list of codecs in
    AppRTCMobile.

BUG=webrtc:7924
R=magjed@webrtc.org

Review-Url: https://codereview.webrtc.org/2966023002 .
Cr-Commit-Position: refs/heads/master@{#18928}

webrtc/sdk/objc/Framework/Headers/WebRTC/WebRTC.h

@@ -46,6 +46,9 @@
 #import <WebRTC/RTCSSLAdapter.h>
 #import <WebRTC/RTCSessionDescription.h>
 #import <WebRTC/RTCTracing.h>
+#import <WebRTC/RTCVideoCodec.h>
+#import <WebRTC/RTCVideoCodecFactory.h>
+#import <WebRTC/RTCVideoCodecH264.h>
 #import <WebRTC/RTCVideoFrame.h>
 #import <WebRTC/RTCVideoRenderer.h>
 #import <WebRTC/RTCVideoSource.h>