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/RTCICECandidate+JSON.h

@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#import "webrtc/api/objc/RTCIceCandidate.h"
+#import "WebRTC/RTCIceCandidate.h"
 
 @interface RTCIceCandidate (JSON)