Add TRACE_EVENTX() in WebAudio rendering pipe line

This CL adds multiple trace events (TRACE_EVENTX) in various points in
the rendering to investigate the cause of glitches.

("webaudio", "AudioDestination::Start")
("webaudio", "AudioDestination::Stop")
("webaudio", "AudioDestination::RequestRenderOnWebThread", "frames_to_render", frames_to_render)
("webaudio", "AudioDestination::Render", "callback_buffer_size", number_of_frames)
("webaudio", "AudioDestinationHandler::Render")

BUG=718554

Review-Url: https://codereview.chromium.org/2858223003
Cr-Commit-Position: refs/heads/master@{#469557}

third_party/WebKit/Source/modules/webaudio/AudioDestinationNode.cpp

@@ -29,6 +29,7 @@
 #include "modules/webaudio/BaseAudioContext.h"
 #include "platform/audio/AudioUtilities.h"
 #include "platform/audio/DenormalDisabler.h"
+#include "platform/instrumentation/tracing/TraceEvent.h"
 #include "platform/wtf/Atomics.h"
 
 namespace blink {
@@ -46,6 +47,8 @@ void AudioDestinationHandler::Render(AudioBus* source_bus,
                                      AudioBus* destination_bus,
                                      size_t number_of_frames,
                                      const AudioIOPosition& output_position) {
+  TRACE_EVENT0("webaudio", "AudioDestinationHandler::Render");
+
   // We don't want denormals slowing down any of the audio processing
   // since they can very seriously hurt performance.  This will take care of all
   // AudioNodes because they all process within this scope.

third_party/WebKit/Source/platform/audio/AudioDestination.cpp

@@ -34,6 +34,7 @@
 #include "platform/WebTaskRunner.h"
 #include "platform/audio/AudioUtilities.h"
 #include "platform/audio/PushPullFIFO.h"
+#include "platform/instrumentation/tracing/TraceEvent.h"
 #include "platform/weborigin/SecurityOrigin.h"
 #include "platform/wtf/PtrUtil.h"
 #include "public/platform/Platform.h"
@@ -100,6 +101,9 @@ void AudioDestination::Render(const WebVector<float*>& destination_data,
                               double delay,
                               double delay_timestamp,
                               size_t prior_frames_skipped) {
+  TRACE_EVENT1("webaudio", "AudioDestination::Render",
+               "callback_buffer_size", number_of_frames);
+
   // This method is called by AudioDeviceThread.
   DCHECK(!IsRenderingThread());
 
@@ -135,6 +139,9 @@ void AudioDestination::RequestRenderOnWebThread(size_t frames_requested,
                                                 double delay,
                                                 double delay_timestamp,
                                                 size_t prior_frames_skipped) {
+  TRACE_EVENT1("webaudio", "AudioDestination::RequestRenderOnWebThread",
+               "frames_to_render", frames_to_render);
+
   // This method is called by WebThread.
   DCHECK(IsRenderingThread());
 
@@ -176,6 +183,7 @@ void AudioDestination::Start() {
 
   // Start the "audio device" after the rendering thread is ready.
   if (web_audio_device_ && !is_playing_) {
+    TRACE_EVENT0("webaudio", "AudioDestination::Start");
     rendering_thread_ =
         Platform::Current()->CreateThread("WebAudio Rendering Thread");
     web_audio_device_->Start();
@@ -189,6 +197,7 @@ void AudioDestination::Stop() {
   // This assumes stopping the "audio device" is synchronous and dumping the
   // rendering thread is safe after that.
   if (web_audio_device_ && is_playing_) {
+    TRACE_EVENT0("webaudio", "AudioDestination::Stop");
     web_audio_device_->Stop();
     rendering_thread_.reset();
     is_playing_ = false;