tracing: Add macros for generating context trace events

third_party/WebKit/Source/platform/TraceEvent.h

Index: third_party/WebKit/Source/platform/TraceEvent.h
diff --git a/third_party/WebKit/Source/platform/TraceEvent.h b/third_party/WebKit/Source/platform/TraceEvent.h
index 01e4edec905443a96d6e8b1fb8a00d14e232de0e..fe656337b7594000f2d31e0b2b7abc0112a0ce6a 100644
--- a/third_party/WebKit/Source/platform/TraceEvent.h
+++ b/third_party/WebKit/Source/platform/TraceEvent.h
@@ -235,6 +235,10 @@
         } \
     } while (0)
 
+// Implementation detail: internal macro to enter and leave a context based on
+// the current scope.
+#define INTERNAL_TRACE_EVENT_SCOPED_CONTEXT(categoryGroup, name, context) \
+    blink::TraceEvent::TraceScopedContext INTERNAL_TRACE_EVENT_UID(scopedTracer)(categoryGroup, name, context)
 
 // These values must be in sync with base::debug::TraceLog::CategoryGroupEnabledFlags.
 #define ENABLED_FOR_RECORDING (1 << 0)
@@ -623,6 +627,31 @@ private:
     IDType m_id;
 };
 
+using TraceContext = const void*;
+
+class TraceScopedContext {
+    WTF_MAKE_NONCOPYABLE(TraceScopedContext);
+
+public:
+    TraceScopedContext(const char* categoryGroup, const char* name, TraceContext context)
+        : m_categoryGroup(categoryGroup)
+        , m_name(name)
+        , m_context(context)
+    {
+        TRACE_EVENT_ENTER_CONTEXT(m_categoryGroup, m_name, m_context);
+    }
+
+    ~TraceScopedContext()
+    {
+        TRACE_EVENT_LEAVE_CONTEXT(m_categoryGroup, m_name, m_context);
+    }
+
+private:
+    const char* m_categoryGroup;
+    const char* m_name;
+    TraceContext m_context;
+};
+
 } // namespace TraceEvent
 
 } // namespace blink