Add method on AVFoundation capturer to adapt output format.

This CL makes a method available on the AVFoundationVideoCapturer
that adapts the output format of captured video to the specified
width and height.

BUG=webrtc:6753

Review-Url: https://codereview.webrtc.org/2528493004
Cr-Commit-Position: refs/heads/master@{#15351}

webrtc/sdk/objc/Framework/Classes/avfoundationvideocapturer.h

@@ -58,6 +58,10 @@ class AVFoundationVideoCapturer : public cricket::VideoCapturer {
   void CaptureSampleBuffer(CMSampleBufferRef sample_buffer,
                            webrtc::VideoRotation rotation);
 
+  // Called to adjust the size of output frames to supplied |width| and
+  // |height|. Also drops frames to make the output match |fps|.
+  void AdaptOutputFormat(int width, int height, int fps);
+
  private:
   RTCAVFoundationVideoCapturerInternal *_capturer;
   webrtc::I420BufferPool _buffer_pool;