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..0ef9fecf85e28afdd44d67bb42419b14e3d07cce 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());

[foolip, 2016/08/08 19:59:11]

This ASSERT should still hold, right, or why was it removed?

[Pat Meenan, 2016/08/09 19:06:32]

UseCounter::count() is exactly this assert + the recordMeasurement() call. 
Instead of duplicating both of them I changed it to pass through to the count()
entry point and leave the logic there.

This isn't necessary for my change since I moved the tracing into the deeper
recordMeasurement() call anyway but came out as part of the cleanup from earlier
iterations.

[foolip, 2016/08/11 08:03:44]

Acknowledged.

-    host->useCounter().recordMeasurement(feature);
+    host->useCounter().count(feature);
 }
 
 void UseCounter::count(const Document& document, Feature feature)
@@ -751,6 +751,7 @@ 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);
 }
 
@@ -783,4 +784,12 @@ UseCounter* UseCounter::getFrom(const StyleSheetContents* sheetContents)
     return 0;
 }
 
+void UseCounter::recordMeasurement(Feature feature)
+{
+    if (!m_muteCount) {

[foolip, 2016/08/08 19:59:11]

Can you do an early return instead, and put this somewhere around
unmuteForInspector/updateMeasurements to be closer to the header file's order?

[Pat Meenan, 2016/08/09 19:06:32]

Done.

+        m_countBits.recordMeasurement(feature);
+        TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("blink.feature_usage"), "FeatureUsed", "feature", feature);
+    }
+}
+
 } // namespace blink