enable / disable Long Task instrumentation based on presence of LongTask observer

third_party/WebKit/Source/core/timing/Performance.cpp

Index: third_party/WebKit/Source/core/timing/Performance.cpp
diff --git a/third_party/WebKit/Source/core/timing/Performance.cpp b/third_party/WebKit/Source/core/timing/Performance.cpp
index 8b236f6a17d02988229c54ca22ea9e3099acbee9..82ba53ff931730f14e98fce4a060d09eaf4944dc 100644
--- a/third_party/WebKit/Source/core/timing/Performance.cpp
+++ b/third_party/WebKit/Source/core/timing/Performance.cpp
@@ -31,8 +31,11 @@
 
 #include "core/timing/Performance.h"
 
+#include "core/InstrumentingAgents.h"
 #include "core/dom/Document.h"
 #include "core/frame/LocalFrame.h"
+#include "core/inspector/InspectedFrames.h"
+#include "core/inspector/InspectorWebPerfAgent.h"
 #include "core/loader/DocumentLoader.h"
 #include "core/timing/PerformanceTiming.h"
 
@@ -92,10 +95,32 @@ PerformanceTiming* Performance::timing() const
     return m_timing.get();
 }
 
+void Performance::enableLongTaskInstrumentation() const
+{
+    if (hasObserverFor(PerformanceEntry::LongTask)

[caseq, 2016/09/15 23:29:40]

nit: we tend to prefer early bail-outs,
i.e. "if (!condition) return" as opposed to "if (condition) { ... }"

[panicker, 2016/09/16 17:07:58]

Ok. Done.

+        && !m_longTaskInspectorAgent) {
+        m_longTaskInspectorAgent = new InspectorWebPerfAgent(
+            InspectedFrames::create(frame()));
+        frame()->instrumentingAgents()->addInspectorWebPerfAgent(
+        m_longTaskInspectorAgent);
+    }
+}
+
+void Performance::disableLongTaskInstrumentation() const
+{
+    if (!hasObserverFor(PerformanceEntry::LongTask)
+        && m_longTaskInspectorAgent) {
+        frame()->instrumentingAgents()->removeInspectorWebPerfAgent(
+            m_longTaskInspectorAgent);
+        m_longTaskInspectorAgent = nullptr;
+    }
+}
+
 DEFINE_TRACE(Performance)
 {
     visitor->trace(m_navigation);
     visitor->trace(m_timing);
+    visitor->trace(m_longTaskInspectorAgent);
     DOMWindowProperty::trace(visitor);
     PerformanceBase::trace(visitor);
 }