Add UMA histograms for WebAudio

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

Index: third_party/WebKit/Source/modules/webaudio/PannerNode.cpp
diff --git a/third_party/WebKit/Source/modules/webaudio/PannerNode.cpp b/third_party/WebKit/Source/modules/webaudio/PannerNode.cpp
index 3318bed2d9b4e11b88722f0b46198ec5a42707a6..44ccd79bcc05a664090b230599995d2bc951e867 100644
--- a/third_party/WebKit/Source/modules/webaudio/PannerNode.cpp
+++ b/third_party/WebKit/Source/modules/webaudio/PannerNode.cpp
@@ -31,6 +31,7 @@
 #include "modules/webaudio/AudioBufferSourceNode.h"
 #include "modules/webaudio/AudioNodeInput.h"
 #include "modules/webaudio/AudioNodeOutput.h"
+#include "platform/Histogram.h"
 #include "platform/audio/HRTFPanner.h"
 #include "wtf/MathExtras.h"
 
@@ -52,7 +53,6 @@ PannerHandler::PannerHandler(
     AudioParamHandler& orientationZ)
     : AudioHandler(NodeTypePanner, node, sampleRate)
     , m_listener(node.context()->listener())
-    , m_panningModel(Panner::PanningModelEqualPower)
     , m_distanceModel(DistanceEffect::ModelInverse)
     , m_isAzimuthElevationDirty(true)
     , m_isDistanceConeGainDirty(true)
@@ -79,6 +79,10 @@ PannerHandler::PannerHandler(
     m_channelCountMode = ClampedMax;
     m_channelInterpretation = AudioBus::Speakers;
 
+    // Explicitly set the default panning model here so that the histograms
+    // include the default value.
+    setPanningModel("equalpower");
+
     initialize();
 }
 
@@ -284,6 +288,10 @@ void PannerHandler::setPanningModel(const String& model)
 
 bool PannerHandler::setPanningModel(unsigned model)
 {
+    DEFINE_STATIC_LOCAL(EnumerationHistogram, panningModelHistogram,
+        ("WebAudio.PannerNode.PanningModel", 2));
+    panningModelHistogram.count(model);

[Mark P, 2016/06/03 19:52:37]

Please put a comment in
third_party/WebKit/Source/platform/audio/Panner.h
by the definition that values are written to logs and should not be changed.

[Raymond Toy, 2016/06/09 19:17:10]

Done.

+
     switch (model) {
     case Panner::PanningModelEqualPower:
     case Panner::PanningModelHRTF: