iOS: Add ability to specify audio constraints.

NOTRY=True
BUG=

Review-Url: https://codereview.webrtc.org/2290583003
Cr-Commit-Position: refs/heads/master@{#13976}

webrtc/sdk/objc/Framework/Classes/RTCMediaSource+Private.h

@@ -0,0 +1,42 @@
+/*
+ *  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.
+ */
+
+#import "WebRTC/RTCMediaSource.h"
+
+#include "webrtc/api/mediastreaminterface.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+typedef NS_ENUM(NSInteger, RTCMediaSourceType) {
+  RTCMediaSourceTypeAudio,
+  RTCMediaSourceTypeVideo,
+};
+
+@interface RTCMediaSource ()
+
+@property(nonatomic, readonly)
+    rtc::scoped_refptr<webrtc::MediaSourceInterface> nativeMediaSource;
+
+- (instancetype)initWithNativeMediaSource:
+   (rtc::scoped_refptr<webrtc::MediaSourceInterface>)nativeMediaSource
+                                     type:(RTCMediaSourceType)type
+    NS_DESIGNATED_INITIALIZER;
+
++ (webrtc::MediaSourceInterface::SourceState)nativeSourceStateForState:
+    (RTCSourceState)state;
+
++ (RTCSourceState)sourceStateForNativeState:
+    (webrtc::MediaSourceInterface::SourceState)nativeState;
+
++ (NSString *)stringForState:(RTCSourceState)state;
+
+@end
+
+NS_ASSUME_NONNULL_END