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.mm

@@ -1,33 +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 "NSString+StdString.h"
-
-@implementation NSString (StdString)
-
-- (std::string)stdString {
-  return [NSString stdStringForString:self];
-}
-
-+ (std::string)stdStringForString:(NSString *)nsString {
-  NSData *charData = [nsString dataUsingEncoding:NSUTF8StringEncoding];
-  return std::string(reinterpret_cast<const char *>(charData.bytes),
-                     charData.length);
-}
-
-+ (NSString *)stringForStdString:(const std::string&)stdString {
-  // std::string may contain null termination character so we construct
-  // using length.
-  return [[NSString alloc] initWithBytes:stdString.data()
-                                  length:stdString.length()
-                                encoding:NSUTF8StringEncoding];
-}
-
-@end