Add stats overlay to iOS AppRTCDemo.

webrtc/examples/objc/AppRTCDemo/ARDBitrateTracker.h

+/*
+ *  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 <Foundation/Foundation.h>
+
+/** Class used to estimate bitrate based on byte count. It is expected that
+ *  byte count is monotonocially increasing. This class tracks the times that
+ *  byte count is updated, and measures the bitrate based on the byte difference
+ *  over the interval between updates.
+ */
+@interface ARDBitrateTracker : NSObject
+
+/** The bitrate in bits per second. */
+@property(nonatomic, readonly) double bitrate;
+/** The bitrate as a formatted string in bps, Kbps or Mbps. */
+@property(nonatomic, readonly) NSString *bitrateString;
+
+/** Converts the bitrate to a readable format in bps, Kbps or Mbps. */
++ (NSString *)bitrateStringForBitrate:(double)bitrate;
+/** Updates the tracked bitrate with the new byte count. */
+- (void)updateBitrateWithCurrentByteCount:(NSInteger)byteCount;
+
+@end