Resolves issue with bad audio using BT headsets on iOS

webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm

Index: webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm
diff --git a/webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm b/webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm
index 7ef5110f476a9cf950eaa3cda86dcb721e3ae6e1..acd4bc2429f0341c0b2a67218d75e88c0ac28244 100644
--- a/webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm
+++ b/webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm
@@ -636,6 +636,31 @@ NSInteger const kRTCAudioSessionErrorConfiguration = -2;
     return NO;
   }
 
+  // It can happen (e.g. in combination with BT devices) that the attempt to set
+  // the preferred sample rate for WebRTC (48kHz) fails. If so, make a new
+  // configuration attempt using the sample rate that worked using the active
+  // audio session. A typical case is that only 8 or 16kHz can be set, e.g. in
+  // combination with BT headsets. Using this "trick" seems to avoid a state
+  // where Core Audio asks for a different number of audio frames than what the
+  // session's I/O buffer duration corresponds to.
+  // TODO(henrika): this fix resolves bugs.webrtc.org/6004 but it has only been
+  // tested on a limited set of iOS devices and BT devices.
+  double session_sample_rate = self.sampleRate;

[tkchin_webrtc, 2016/06/15 21:21:46]

in ObjC methods, use ObjC style rules. Camel case instead of underscore.

e.g sessionSampleRate 

[henrika_webrtc, 2016/06/16 12:12:26]

Done.

+  double preferred_sample_rate = webRTCConfig.sampleRate;
+  if (session_sample_rate != preferred_sample_rate) {
+    RTCLogWarning(

[tkchin_webrtc, 2016/06/15 21:21:46]

fyi I've made it to that ObjC files are now 100char (consistent with ObjC style)

[henrika_webrtc, 2016/06/16 12:12:26]

Got it. Keeping 80 here to be consistent with the rest of the file.

+        @"Current sample rate (%.2f) is not the preferred rate (%.2f)",
+        session_sample_rate, preferred_sample_rate);
+    if (![self setPreferredSampleRate:session_sample_rate
+                                error:&error]) {
+      RTCLogError(@"Failed to set preferred sample rate: %@",
+                  error.localizedDescription);
+      if (outError) {
+        *outError = error;
+      }
+    }
+  }
+
   return YES;
 }