Instrument AudioContext.baseLatency for identifiability study.

Bug: 973801
Change-Id: Ic3a1b125ffa72b282899c2e65b7dacb43af13cfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354589
Commit-Queue: Russ Hamilton <behamilton@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798015}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -2714,6 +2714,7 @@ enum WebFeature {
   kV8FontMetadata_GetTables_Method = 3384,
   kV8FontMetadata_Blob_Method = 3385,
   kV8FontManager_Query_Method = 3386,
+  kAudioContextBaseLatency = 3387,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/modules/webaudio/audio_context.idl

@@ -45,7 +45,7 @@ enum AudioContextLatencyCategory {
 
     // Number of seconds of processing latency incurred by the AudioContext
     // passing the audio from the AudioDestinationNode to the audio subsystem
-    readonly attribute double baseLatency;
+    [HighEntropy=Direct, MeasureAs=AudioContextBaseLatency] readonly attribute double baseLatency;
 
     [RaisesException, MeasureAs=AudioContextCreateMediaElementSource] MediaElementAudioSourceNode createMediaElementSource(HTMLMediaElement mediaElement);
     [RaisesException, MeasureAs=AudioContextCreateMediaStreamSource] MediaStreamAudioSourceNode createMediaStreamSource(MediaStream mediaStream);

tools/metrics/histograms/enums.xml

@@ -28725,6 +28725,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3384" label="V8FontMetadata_GetTables_Method"/>
   <int value="3385" label="V8FontMetadata_Blob_Method"/>
   <int value="3386" label="V8FontManager_Query_Method"/>
+  <int value="3387" label="AudioContextBaseLatency"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">