Update/move RTCVideoRendererAdapter to webrtc/api/objc.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1533323003 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Commit-Position: refs/heads/master@{#11347}
diff --git a/webrtc/api/api.gyp b/webrtc/api/api.gyp
index f957f5b..ec88a3c 100644
--- a/webrtc/api/api.gyp
+++ b/webrtc/api/api.gyp
@@ -46,6 +46,9 @@
             'objc/RTCVideoFrame.h',
             'objc/RTCVideoFrame.mm',
             'objc/RTCVideoRenderer.h',
+            'objc/RTCVideoRendererAdapter+Private.h',
+            'objc/RTCVideoRendererAdapter.h',
+            'objc/RTCVideoRendererAdapter.mm',
             'objc/RTCVideoSource+Private.h',
             'objc/RTCVideoSource.h',
             'objc/RTCVideoSource.mm',