Reland of Injectable Obj-C video codecs (patchset #1 id:1 of https://codereview.webrtc.org/2979973002/ )

Reason for revert:
Fix the broken build file

Original issue's description:
> Revert of Injectable Obj-C video codecs (patchset #3 id:400001 of https://codereview.webrtc.org/2981583002/ )
>
> Reason for revert:
> Breaks bots. Build file incorrect.
>
> Original issue's description:
> > 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}
> > Committed: a5f1de1e65
>
> TBR=magjed@webrtc.org,tkchin@webrtc.org,jtteh@webrtc.org,andersc@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/2979973002
> Cr-Commit-Position: refs/heads/master@{#19004}
> Committed: 81d40ee149

TBR=magjed@webrtc.org,tkchin@webrtc.org,jtteh@webrtc.org,sprang@webrtc.org
BUG=webrtc:7924

Review-Url: https://codereview.webrtc.org/2979983002
Cr-Commit-Position: refs/heads/master@{#19005}

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>