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/RTCAudioSource+Private.h

@@ -0,0 +1,32 @@
+/*
+ *  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/RTCAudioSource.h"
+
+#import "RTCMediaSource+Private.h"
+
+@interface RTCAudioSource ()
+
+/**
+ * The AudioSourceInterface object passed to this RTCAudioSource during
+ * construction.
+ */
+@property(nonatomic, readonly) rtc::scoped_refptr<webrtc::AudioSourceInterface> nativeAudioSource;
+
+/** Initialize an RTCAudioSource from a native AudioSourceInterface. */
+- (instancetype)initWithNativeAudioSource:
+    (rtc::scoped_refptr<webrtc::AudioSourceInterface>)nativeAudioSource
+    NS_DESIGNATED_INITIALIZER;
+
+- (instancetype)initWithNativeMediaSource:
+    (rtc::scoped_refptr<webrtc::MediaSourceInterface>)nativeMediaSource
+                                     type:(RTCMediaSourceType)type NS_UNAVAILABLE;
+
+@end