Remove use of PCH files in webrtc/sdk/objc

A decision has been made to not use prefix header files.

BUG=None

Review-Url: https://codereview.webrtc.org/2590823002
Cr-Commit-Position: refs/heads/master@{#16056}

webrtc/sdk/objc/Framework/Classes/RTCShader.h

@@ -8,6 +8,8 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#import <Foundation/Foundation.h>
+
 #import "RTCOpenGLDefines.h"
 
 @class RTCVideoFrame;