Reland of Injectable Obj-C video codecs (patchset #1 id:1 of https://codereview.webrtc.org/2975963002/ )

Reason for revert:
New CL for fixing the issues

Original issue's description:
> Revert of Injectable Obj-C video codecs (patchset #8 id:140001 of https://codereview.webrtc.org/2966023002/ )
>
> Reason for revert:
> Causes no video in certain scenarios. Please come up with a test plan or unit test to prevent such problems in the future.
>
> Original issue's description:
> > 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}
> > Committed: a0349c138d
>
> TBR=magjed@webrtc.org,andersc@webrtc.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=webrtc:7924
> NOTRY=true
>
> Review-Url: https://codereview.webrtc.org/2975963002
> Cr-Commit-Position: refs/heads/master@{#18979}
> Committed: 1095ada7ad

R=magjed@webrtc.org
TBR=tkchin@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7924

Review-Url: https://codereview.webrtc.org/2981583002 .
Cr-Commit-Position: refs/heads/master@{#19002}

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

@@ -47,6 +47,9 @@
 #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>