Adding more detail to MessageQueue::Dispatch logging.

Every message will now be traced with the location from which it was
posted, including function name, file and line number.

This CL also writes a normal LOG message when the dispatch took more
than a certain amount of time (currently 50ms).

This logging should help us identify messages that are taking
longer than expected to be dispatched.

R=pthatcher@webrtc.org, tommi@webrtc.org

Review URL: https://codereview.webrtc.org/2019423006 .

Cr-Original-Commit-Position: refs/heads/master@{#13104}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5d97a9a05b928ac50e14a0e4d8aaa42298cad056
diff --git a/app/webrtc/objc/avfoundationvideocapturer.mm b/app/webrtc/objc/avfoundationvideocapturer.mm
index 2b1c245..d525217 100644
--- a/app/webrtc/objc/avfoundationvideocapturer.mm
+++ b/app/webrtc/objc/avfoundationvideocapturer.mm
@@ -487,7 +487,7 @@
   // after it has successfully been signaled.
   CVBufferRetain(image_buffer);
   AVFoundationFrame frame(image_buffer, rtc::TimeNanos());
-  _startThread->Post(this, kMessageTypeFrame,
+  _startThread->Post(RTC_FROM_HERE, this, kMessageTypeFrame,
                      new rtc::TypedMessageData<AVFoundationFrame>(frame));
 }