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/Headers/WebRTC/RTCAVFoundationVideoSource.h

@@ -29,6 +29,16 @@ RTC_EXPORT
 
 - (instancetype)init NS_UNAVAILABLE;
 
+/**
+ * Calling this function will cause frames to be scaled down to the
+ * requested resolution. Also, frames will be cropped to match the
+ * requested aspect ratio, and frames will be dropped to match the
+ * requested fps. The requested aspect ratio is orientation agnostic and
+ * will be adjusted to maintain the input orientation, so it doesn't
+ * matter if e.g. 1280x720 or 720x1280 is requested.
+ */
+- (void)adaptOutputFormatToWidth:(int)width height:(int)height fps:(int)fps;
+
 /** Returns whether rear-facing camera is available for use. */
 @property(nonatomic, readonly) BOOL canUseBackCamera;