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/RTCAVFoundationVideoSource.mm

@@ -32,6 +32,12 @@
   return [super initWithNativeVideoSource:source];
 }
 
+- (void)adaptOutputFormatToWidth:(int)width
+                          height:(int)height
+                             fps:(int)fps {
+  self.capturer->AdaptOutputFormat(width, height, fps);
+}
+
 - (BOOL)canUseBackCamera {
   return self.capturer->CanUseBackCamera();
 }

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;

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

@@ -113,6 +113,11 @@ bool AVFoundationVideoCapturer::GetUseBackCamera() const {
   return _capturer.useBackCamera;
 }
 
+void AVFoundationVideoCapturer::AdaptOutputFormat(int width, int height, int fps) {
+  cricket::VideoFormat format(width, height, cricket::VideoFormat::FpsToInterval(fps), 0);
+  video_adapter()->OnOutputFormatRequest(format);
+}
+
 void AVFoundationVideoCapturer::CaptureSampleBuffer(
     CMSampleBufferRef sample_buffer, VideoRotation rotation) {
   if (CMSampleBufferGetNumSamples(sample_buffer) != 1 ||