Add a sampling tracing to RenderWidget::DoDeferredUpdate

base/debug/trace_event.h

Index: base/debug/trace_event.h
diff --git a/base/debug/trace_event.h b/base/debug/trace_event.h
index aa4cc759b79eeabca051ff795c07455cf56eb3c2..2bb04061ea63fd993d328f279fd89e57eb9e6afc 100644
--- a/base/debug/trace_event.h
+++ b/base/debug/trace_event.h
@@ -299,9 +299,16 @@
 // want the inconsistency/expense of storing two pointers.
 // |thread_bucket| is [0..2] and is used to statically isolate samples in one
 // thread from others.
-#define TRACE_EVENT_SAMPLE_STATE(thread_bucket, category, name)                \
-  TRACE_EVENT_API_ATOMIC_STORE(                                                \
-      TRACE_EVENT_API_THREAD_BUCKET(thread_bucket),                            \
+#define TRACE_EVENT_SAMPLING_STATE0(category, name) \
+    INTERNAL_TRACE_EVENT_SAMPLING_STATE(0, category, name)
+#define TRACE_EVENT_SAMPLING_STATE1(category, name) \
+    INTERNAL_TRACE_EVENT_SAMPLING_STATE(1, category, name)
+#define TRACE_EVENT_SAMPLING_STATE2(category, name) \
+    INTERNAL_TRACE_EVENT_SAMPLING_STATE(2, category, name)
+
+#define INTERNAL_TRACE_EVENT_SAMPLING_STATE(thread_bucket, category, name) \
+  TRACE_EVENT_API_ATOMIC_STORE(                                            \
+      TRACE_EVENT_API_THREAD_BUCKET(thread_bucket),                        \
       reinterpret_cast<TRACE_EVENT_API_ATOMIC_WORD>(category "\0" name));
 
 // Records a single BEGIN event called "name" immediately, with 0, 1 or 2
@@ -763,6 +770,40 @@ TRACE_EVENT_API_CLASS_EXPORT extern TRACE_EVENT_API_ATOMIC_WORD g_trace_state1;
 TRACE_EVENT_API_CLASS_EXPORT extern TRACE_EVENT_API_ATOMIC_WORD g_trace_state2;
 #define TRACE_EVENT_API_THREAD_BUCKET(thread_bucket)                           \
   g_trace_state##thread_bucket
+
+// TraceEventSamplingState0Scope records the current sampling state
+// and sets a new sampling state. When the scope exists, it restores
+// the sampling state having recorded.
+class TraceEventSamplingState0Scope {
+public:
+    TraceEventSamplingState0Scope(const char* category_and_name)
+    {
+        previousState0_ = TraceEventSamplingState0Scope::current();
+        TraceEventSamplingState0Scope::forceCurrent(category_and_name);
+    }
+
+    ~TraceEventSamplingState0Scope()
+    {
+        TraceEventSamplingState0Scope::forceCurrent(previousState0_);
+    }
+
+    static inline const char* current()

[scottmg, 2013/07/03 02:16:10]

In chromium code, these should be Current()

[haraken, 2013/07/03 02:38:50]

Done.

+    {
+        return reinterpret_cast<const char*>(TRACE_EVENT_API_ATOMIC_LOAD(
+          TRACE_EVENT_API_THREAD_BUCKET(0)));
+    }
+
+    static inline void forceCurrent(const char* category_and_name)

[scottmg, 2013/07/03 02:16:10]

and ForceCurrent. Maybe just "Set" would be fine though?

[haraken, 2013/07/03 02:38:50]

Done.

+    {
+        TRACE_EVENT_API_ATOMIC_STORE(
+          TRACE_EVENT_API_THREAD_BUCKET(0),
+          reinterpret_cast<TRACE_EVENT_API_ATOMIC_WORD>(
+            const_cast<char*>(category_and_name)));
+    }
+
+private:
+    const char* previousState0_;

[scottmg, 2013/07/03 02:16:10]

and previous_state0_

[haraken, 2013/07/03 02:38:50]

Done.

+};
 #endif
 
 ////////////////////////////////////////////////////////////////////////////////