Add UMA histogramm for AudioContext.getOutputTimestamp()

Specification: https://webaudio.github.io/web-audio-api/#widl-AudioContext-getOutputTimestamp-AudioTimestamp

BUG=619533

Review-Url: https://codereview.chromium.org/2587223002
Cr-Commit-Position: refs/heads/master@{#439752}

third_party/WebKit/Source/core/frame/UseCounter.h

@@ -1384,6 +1384,7 @@ class CORE_EXPORT UseCounter {
     V8AudioContext_Constructor = 1698,
     V8OfflineAudioContext_Constructor = 1699,
     AppInstalledEventAddListener = 1700,
+    AudioContextGetOutputTimestamp = 1701,
 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.

third_party/WebKit/Source/modules/webaudio/AudioContext.idl

@@ -37,7 +37,7 @@
     [MeasureAs=AudioContextClose, CallWith=ScriptState, ImplementedAs=closeContext] Promise<void> close();
 
     // Output timestamp
-    [CallWith=ScriptState] AudioTimestamp getOutputTimestamp();
+    [MeasureAs=AudioContextGetOutputTimestamp, CallWith=ScriptState] AudioTimestamp getOutputTimestamp();
 
     // Sources
     // TODO(rtoy): The following methods should be here instead of in BaseAudioContext:

tools/metrics/histograms/histograms.xml

@@ -87991,6 +87991,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1698" label="V8AudioContext_Constructor"/>
   <int value="1699" label="V8OfflineAudioContext_Constructor"/>
   <int value="1700" label="AppInstalledEventAddListener"/>
+  <int value="1701" label="AudioContextGetOutputTimestamp"/>
 </enum>
 
 <enum name="FetchRequestMode" type="int">