Compile rtc_base_objc for Mac.

webrtc/build/WebRTC-Prefix.pch

+/*
+ *  Copyright 2016 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.
+ */
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+#if !defined(__has_feature) || !__has_feature(nullability)
+#error "This file requires nullability support."
+#endif
+
+// The following nonnull macros were introduced in OSX SDK 10.10.3. However,
+// the bots appear to be running something older. We define them here if they
+// aren't already defined in NSObjCRuntime.h
+#ifdef __OBJC__
+
+#include <Foundation/NSObjCRuntime.h>
+
+#if !defined(NS_ASSUME_NONNULL_BEGIN)
+#define NS_ASSUME_NONNULL_BEGIN
+#endif
+
+#if !defined(NS_ASSUME_NONNULL_END)
+#define NS_ASSUME_NONNULL_END
+#endif
+
+#endif