Canvas lowLatency: add UseCounter

This CL adds a UseCounter for HTMLCanvas lowLatency use cases, to
complement said experiment, see the feature entry in
https://www.chromestatus.com/feature/6360971442388992

Bug: 895907
Change-Id: I7e81c4f317816b301d0845fc4561c8966000db36
Reviewed-on: https://chromium-review.googlesource.com/c/1293557Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601605}

third_party/blink/public/platform/web_feature.mojom

@@ -2061,6 +2061,7 @@ enum WebFeature {
   kMalformedCSP = 2608,
   kV8AttemptOverrideReadOnlyOnPrototypeSloppy = 2609,
   kV8AttemptOverrideReadOnlyOnPrototypeStrict = 2610,
+  kHTMLCanvasElementLowLatency = 2611,
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.
   // Also, run update_use_counter_feature_enum.py in

third_party/blink/renderer/core/html/canvas/html_canvas_element.cc

@@ -317,6 +317,9 @@ CanvasRenderingContext* HTMLCanvasElement::GetCanvasRenderingContext(
     // We don't actually need the begin frame signal when in low latency mode,
     // but we need to subscribe to it or else dispatching frames will not work.
     frame_dispatcher_->SetNeedsBeginFrame(GetPage()->IsPageVisible());
+
+    UseCounter::Count(GetDocument().GetFrame(),
+                      WebFeature::kHTMLCanvasElementLowLatency);
   }
 
   SetNeedsCompositingUpdate();

tools/metrics/histograms/enums.xml

@@ -20528,6 +20528,7 @@ Called by update_net_error_codes.py.-->
   <int value="2608" label="MalformedCSP"/>
   <int value="2609" label="V8AttemptOverrideReadOnlyOnPrototypeSloppy"/>
   <int value="2610" label="V8AttemptOverrideReadOnlyOnPrototypeStrict"/>
+  <int value="2611" label="HTMLCanvasElementLowLatency"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">