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();
 }