iOS: Add ability to specify audio constraints.

webrtc/sdk/objc/Framework/Classes/RTCAudioSource.mm

+/*
+ *  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 "RTCAudioSource+Private.h"
+
+#include "webrtc/base/checks.h"
+
+@implementation RTCAudioSource {
+  rtc::scoped_refptr<webrtc::AudioSourceInterface> _nativeAudioSource;
+}
+
+- (instancetype)initWithNativeAudioSource:
+    (rtc::scoped_refptr<webrtc::AudioSourceInterface>)nativeAudioSource {
+  RTC_DCHECK(nativeAudioSource);
+  if (self = [super initWithNativeMediaSource:nativeAudioSource
+                                         type:RTCMediaSourceTypeAudio]) {
+    _nativeAudioSource = nativeAudioSource;
+  }
+  return self;
+}
+
+- (instancetype)initWithNativeMediaSource:
+    (rtc::scoped_refptr<webrtc::MediaSourceInterface>)nativeMediaSource
+                                     type:(RTCMediaSourceType)type {
+  RTC_NOTREACHED();
+  return nil;
+}
+
+- (NSString *)description {
+  NSString *stateString = [[self class] stringForState:self.state];
+  return [NSString stringWithFormat:@"RTCAudioSource( %p ): %@", self, stateString];
+}
+
+#pragma mark - Private
+
+- (rtc::scoped_refptr<webrtc::AudioSourceInterface>)nativeAudioSource {
+  return _nativeAudioSource;
+}
+
+@end