Build dynamic iOS SDK.

- Places most ObjC code into webrtc/sdk/objc instead.
- New gyp targets to build, strip and export symbols for dylib.
- Removes old script used to generate dylib.

BUG=

Review URL: https://codereview.webrtc.org/1903663002

Cr-Commit-Position: refs/heads/master@{#12524}

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

@@ -11,8 +11,9 @@
 #import "APPRTCViewController.h"
 
 #import <AVFoundation/AVFoundation.h>
-#import "webrtc/api/objc/RTCNSGLVideoView.h"
-#import "webrtc/api/objc/RTCVideoTrack.h"
+
+#import "WebRTC/RTCNSGLVideoView.h"
+#import "WebRTC/RTCVideoTrack.h"
 
 #import "ARDAppClient.h"