EventTiming: Add UseCounter to track explicit usage

This CL adds a UseCounter that tracks the two ways to use EventTiming
explicitly:
* getEntriesByType or getEntriesByName with event/firstInput.
* Having PerformanceObservers of event/firstInput.

Bug: 851484
Change-Id: I8c2af71d089dbbf7db87b94b8b5d9f56e1699998
Reviewed-on: https://chromium-review.googlesource.com/1115511Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570539}

third_party/blink/public/platform/web_feature.mojom

@@ -1947,6 +1947,7 @@ enum WebFeature {
   kLegacySymantecCertMainFrameResource = 2484,
   kLegacySymantecCertInSubresource = 2485,
   kLegacySymantecCertInSubframeMainResource = 2486,
+  kEventTimingExplicitlyRequested = 2487,
 
   // 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/timing/performance.cc

@@ -171,10 +171,14 @@ PerformanceEntryVector Performance::getEntriesByType(const String& entry_type) {
         entries.push_back(resource);
       break;
     case PerformanceEntry::kEvent:
+      UseCounter::Count(GetExecutionContext(),
+                        WebFeature::kEventTimingExplicitlyRequested);
       for (const auto& event : event_timing_buffer_)
         entries.push_back(event);
       break;
     case PerformanceEntry::kFirstInput:
+      UseCounter::Count(GetExecutionContext(),
+                        WebFeature::kEventTimingExplicitlyRequested);
       if (first_input_timing_)
         entries.push_back(first_input_timing_);
       break;
@@ -246,11 +250,19 @@ PerformanceEntryVector Performance::getEntriesByName(const String& name,
         entries.push_back(event);
     }
   }
+  if (type == PerformanceEntry::kEvent) {
+    UseCounter::Count(GetExecutionContext(),
+                      WebFeature::kEventTimingExplicitlyRequested);
+  }
 
   if (entry_type.IsNull() || type == PerformanceEntry::kFirstInput) {
     if (first_input_timing_ && first_input_timing_->name() == name)
       entries.push_back(first_input_timing_);
   }
+  if (type == PerformanceEntry::kFirstInput) {
+    UseCounter::Count(GetExecutionContext(),
+                      WebFeature::kEventTimingExplicitlyRequested);
+  }
 
   if (entry_type.IsNull() || type == PerformanceEntry::kNavigation) {
     if (!navigation_timing_)

third_party/blink/renderer/core/timing/window_performance.cc

@@ -371,8 +371,11 @@ void WindowPerformance::ReportEventTimings(WebLayerTreeView::SwapResult result,
     if (duration_in_ms <= kEventTimingDurationThresholdInMs)
       continue;
 
-    if (ObservingEventTimingEntries())
+    if (ObservingEventTimingEntries()) {
+      UseCounter::Count(GetFrame(),
+                        WebFeature::kEventTimingExplicitlyRequested);
       NotifyObserversOfEntry(*entry);
+    }
 
     if (ShouldBufferEventTiming() && !IsEventTimingBufferFull())
       AddEventTimingBuffer(*entry);
@@ -388,8 +391,10 @@ void WindowPerformance::DispatchFirstInputTiming(
   if (!entry)
     return;
   DCHECK_EQ("firstInput", entry->entryType());
-  if (HasObserverFor(PerformanceEntry::kFirstInput))
+  if (HasObserverFor(PerformanceEntry::kFirstInput)) {
+    UseCounter::Count(GetFrame(), WebFeature::kEventTimingExplicitlyRequested);
     NotifyObserversOfEntry(*entry);
+  }
 
   DCHECK(!first_input_timing_);
   if (ShouldBufferEventTiming())

tools/metrics/histograms/enums.xml

@@ -19058,6 +19058,7 @@ Called by update_net_error_codes.py.-->
   <int value="2484" label="LegacySymantecCertMainFrameResource"/>
   <int value="2485" label="LegacySymantecCertInSubresource"/>
   <int value="2486" label="LegacySymantecCertInSubframeMainResource"/>
+  <int value="2487" label="EventTimingExplicitlyRequested"/>
 </enum>
 
 <enum name="FeedbackSource">