Split iOS sdk in to separate targets

This CL splits the iOS sdk into separate static libraries for video,
audio, ui, common, and peerconnection-related code. This will in the
future make it easier to compile WebRTC without unneeded components.

BUG=webrtc:4867

Review-Url: https://codereview.webrtc.org/2862543002
Cr-Commit-Position: refs/heads/master@{#18166}

webrtc/sdk/objc/Framework/Classes/NSString+StdString.h

@@ -1,26 +0,0 @@
-/*
- *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#import <Foundation/Foundation.h>
-
-#include <string>
-
-NS_ASSUME_NONNULL_BEGIN
-
-@interface NSString (StdString)
-
-@property(nonatomic, readonly) std::string stdString;
-
-+ (std::string)stdStringForString:(NSString *)nsString;
-+ (NSString *)stringForStdString:(const std::string&)stdString;
-
-@end
-
-NS_ASSUME_NONNULL_END