Update API for Objective-C RTCMediaSource.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1538263002 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Commit-Position: refs/heads/master@{#11210}
diff --git a/webrtc/api/objc/RTCMediaSource+Private.h b/webrtc/api/objc/RTCMediaSource+Private.h
new file mode 100644
index 0000000..fcbaad8
--- /dev/null
+++ b/webrtc/api/objc/RTCMediaSource+Private.h
@@ -0,0 +1,41 @@
+/*
+ *  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 "RTCMediaSource.h"
+
+#include "talk/app/webrtc/mediastreaminterface.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCMediaSource ()
+
+/**
+ * The MediaSourceInterface object passed to this RTCMediaSource during
+ * construction.
+ */
+@property(nonatomic, readonly)
+    rtc::scoped_refptr<webrtc::MediaSourceInterface> nativeMediaSource;
+
+/** Initialize an RTCMediaSource from a native MediaSourceInterface. */
+- (instancetype)initWithNativeMediaSource:
+    (rtc::scoped_refptr<webrtc::MediaSourceInterface>)nativeMediaSource
+    NS_DESIGNATED_INITIALIZER;
+
++ (webrtc::MediaSourceInterface::SourceState)nativeSourceStateForState:
+    (RTCSourceState)state;
+
++ (RTCSourceState)sourceStateForNativeState:
+    (webrtc::MediaSourceInterface::SourceState)nativeState;
+
++ (NSString *)stringForState:(RTCSourceState)state;
+
+@end
+
+NS_ASSUME_NONNULL_END