Add stats overlay to iOS AppRTCDemo.

webrtc/examples/objc/AppRTCDemo/ARDBitrateTracker.m

+/*
+ *  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 "ARDBitrateTracker.h"
+
+#import <QuartzCore/QuartzCore.h>
+
+@implementation ARDBitrateTracker {
+  CFTimeInterval _prevTime;
+  NSInteger _prevByteCount;
+}
+
+@synthesize bitrate = _bitrate;
+
++ (NSString *)bitrateStringForBitrate:(double)bitrate {
+  if (bitrate > 1e6) {
+    return [NSString stringWithFormat:@"%.2fMbps", bitrate * 1e-6];
+  } else if (bitrate > 1e3) {
+    return [NSString stringWithFormat:@"%.0fKbps", bitrate * 1e-3];
+  } else {
+    return [NSString stringWithFormat:@"%.0fbps", bitrate];
+  }
+}
+
+- (NSString *)bitrateString {
+  return [[self class] bitrateStringForBitrate:_bitrate];
+}
+
+- (void)updateBitrateWithCurrentByteCount:(NSInteger)byteCount {
+  CFTimeInterval currentTime = CACurrentMediaTime();
+  if (_prevTime && (byteCount > _prevByteCount)) {
+    _bitrate = (byteCount - _prevByteCount) * 8 / (currentTime - _prevTime);
+  }
+  _prevByteCount = byteCount;
+  _prevTime = currentTime;
+}
+
+@end