Add a histogram to measure CSS transition cancellation

This CL adds a histogram to measure how often a CSS transition is
cancelled by removing the transition style.

Bug: 934700
Change-Id: I53f608f0e4cf2ec07adaecb879cffbb557ce2753
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1632714Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664323}

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

@@ -2304,6 +2304,7 @@ enum WebFeature {
   kV8XRSession_End_Method = 2911,
   kV8XRWebGLLayer_Constructor = 2912,
   kFetchKeepalive = 2913,
+  kCSSTransitionCancelledByRemovingStyle = 2914,
 
   // 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/core/animation/css/css_animations.cc

@@ -948,6 +948,12 @@ void CSSAnimations::CalculateTransitionUpdate(CSSAnimationUpdate& update,
       if (!any_transition_had_transition_all && !animation_style_recalc &&
           !listed_properties.Contains(property)) {
         update.CancelTransition(property);
+        // Measure how often transitions are cancelled by removing their style.
+        // See https://crbug.com/934700.
+        if (!transition_data) {
+          UseCounter::Count(animating_element->GetDocument(),
+                            WebFeature::kCSSTransitionCancelledByRemovingStyle);
+        }
       } else if (entry.value.animation->FinishedInternal()) {
         update.FinishTransition(property);
       }

tools/metrics/histograms/enums.xml

@@ -23408,6 +23408,7 @@ Called by update_net_error_codes.py.-->
   <int value="2911" label="V8XRSession_End_Method"/>
   <int value="2912" label="V8XRWebGLLayer_Constructor"/>
   <int value="2913" label="FetchKeepalive"/>
+  <int value="2914" label="CSSTransitionCancelledByRemovingStyle"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">