Add RTCAudioSession proxy class.

webrtc/modules/audio_device/ios/objc/RTCAudioSession+Private.h

+/*
+ *  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/modules/audio_device/ios/objc/RTCAudioSession.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCAudioSession ()
+
+/** The lock that guards access to AVAudioSession methods. */
+@property(nonatomic, strong) NSRecursiveLock *lock;
+
+/** The delegates. */
+@property(nonatomic, readonly) NSSet *delegates;
+
+/** Number of times setActive:YES has succeeded without a balanced call to
+ *  setActive:NO.
+ */
+@property(nonatomic, readonly) NSInteger activationCount;
+
+@end
+
+NS_ASSUME_NONNULL_END