iOS/MacOS:Refactor metal rendering by extracting common implementation

in separate super class.

Included changes:
- Fix rendering on iOS to support NV12 and i420 frames
- Improve code style
- Update build targets
- Update tests

BUG=webrtc:7079

Review-Url: https://codereview.webrtc.org/2784243003
Cr-Commit-Position: refs/heads/master@{#17923}

webrtc/examples/objc/AppRTCMobile/mac/APPRTCViewController.m

@@ -216,7 +216,6 @@ static NSUInteger const kBottomViewHeight = 200;
   [_scrollView setDocumentView:_logView];
   [self addSubview:_scrollView];
 
-
 // NOTE (daniela): Ignoring Clang diagonstic here.
 // We're performing run time check to make sure class is available on runtime.
 // If not we're providing sensible default.