Added trace events for blink feature usage

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

Index: third_party/WebKit/Source/core/frame/UseCounter.cpp
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.cpp b/third_party/WebKit/Source/core/frame/UseCounter.cpp
index 3f8b28145a80dd664e03efecd65de04f7d4df42d..6582251903466e24d921f6ed76fd0d26ed014801 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.cpp
+++ b/third_party/WebKit/Source/core/frame/UseCounter.cpp
@@ -36,6 +36,7 @@
 #include "core/inspector/ConsoleMessage.h"
 #include "core/workers/WorkerGlobalScope.h"
 #include "platform/Histogram.h"
+#include "platform/TraceEvent.h"
 
 namespace blink {
 
@@ -662,8 +663,7 @@ void UseCounter::count(const Frame* frame, Feature feature)
     if (!host)
         return;
 
-    ASSERT(Deprecation::deprecationMessage(feature).isEmpty());
-    host->useCounter().recordMeasurement(feature);
+    host->useCounter().count(feature);
 }
 
 void UseCounter::count(const Document& document, Feature feature)
@@ -751,12 +751,14 @@ void UseCounter::count(CSSParserMode cssParserMode, CSSPropertyID feature)
     if (!isUseCounterEnabledForMode(cssParserMode) || m_muteCount)
         return;
 
+    TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("blink.feature_usage"), "CSSFeatureUsed", "feature", feature);
     m_CSSFeatureBits.quickSet(feature);
 }
 
 void UseCounter::count(Feature feature)
 {
     ASSERT(Deprecation::deprecationMessage(feature).isEmpty());
+    TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("blink.feature_usage"), "FeatureUsed", "feature", feature);

[foolip, 2016/08/03 14:53:27]

This relates to
https://codereview.chromium.org/2203913002/diff/1/third_party/WebKit/Source/c...

Deprecation::countDeprecation doesn't use this code path, so it looks like
deprecated things will be missed here. And it first checks
hasRecordedMeasurement(feature) to avoid double logging to the console, so some
care is needed to not just trace once for deprecations but any number of times
for the rest.

What happens if you try to push down the tracing into
UseCounter::recordMeasurement inside the m_muteCount guard and also check
hasRecordedMeasurement there?

Something like that would be a closer fit for the above change, where you trace
after some guards and right before the poking and the bit vector.

[Pat Meenan, 2016/08/03 18:43:25]

Done.

     recordMeasurement(feature);
 }