Compile rtc_api_objc on Mac.

webrtc/api/objctests/RTCMediaConstraintsTest.mm

 /*
  *  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.
  */
 
@@ skipping 29 matching lines @@
       nativeConstraints->GetOptional();
   [self expectConstraints:optional inNativeConstraints:nativeOptional];
 }
 
 - (void)expectConstraints:(NSDictionary *)constraints
       inNativeConstraints:
     (webrtc::MediaConstraintsInterface::Constraints)nativeConstraints {
   EXPECT_EQ(constraints.count, nativeConstraints.size());
 
   for (NSString *key in constraints) {
-    NSString *value = constraints[key];
+    NSString *value = [constraints objectForKey:key];
 
     std::string nativeValue;
     bool found = nativeConstraints.FindFirst(key.stdString, &nativeValue);
     EXPECT_TRUE(found);
     EXPECT_EQ(value.stdString, nativeValue);
   }
 }
 
 @end
 
 TEST(RTCMediaConstraintsTest, MediaConstraintsTest) {
   @autoreleasepool {
     RTCMediaConstraintsTest *test = [[RTCMediaConstraintsTest alloc] init];
     [test testMediaConstraints];
   }
 }