tracing: Add macros for generating context trace events

base/trace_event/trace_event.h

Index: base/trace_event/trace_event.h
diff --git a/base/trace_event/trace_event.h b/base/trace_event/trace_event.h
index 7a2a65653d22c88bd9cc70f5f8998d760a60c462..f804e163e2f0f8f555e49ddd5947e6a951a1ee63 100644
--- a/base/trace_event/trace_event.h
+++ b/base/trace_event/trace_event.h
@@ -12,6 +12,7 @@
 #include <string>
 
 #include "base/atomicops.h"
+#include "base/macros.h"
 #include "base/time/time.h"
 #include "base/trace_event/common/trace_event_common.h"
 #include "base/trace_event/trace_event_system_stats_monitor.h"
@@ -335,6 +336,11 @@ TRACE_EVENT_API_CLASS_EXPORT extern \
       } \
     } while (0)
 
+// Implementation detail: internal macro to enter and leave a context based on
+// the current scope.
+#define INTERNAL_TRACE_EVENT_SCOPED_CONTEXT(category_group, name, context) \
+  trace_event_internal::TraceScopedContext INTERNAL_TRACE_EVENT_UID(       \
+      scoped_trace)(category_group, name, context)

[fmeawad, 2016/01/19 18:38:10]

Can you move this to common as well, I do not see anything platform specific in
the TraceScopedContext and it would make it easier to start using everywhere?

[Sami, 2016/01/20 10:34:08]

Yeah, I wasn't sure where we draw the line. I didn't see any classes or
namespace declarations in common -- does it make sense to start declaring these
kind of things there?

TraceScopeContext uses DISALLOW_COPY_AND_ASSIGN which is kinda sorta platform
specific.

 
 namespace trace_event_internal {
 
@@ -897,11 +903,35 @@ class TraceEventSamplingStateScope {
   const char* previous_state_;
 };
 
+using TraceContext = const void*;
+
+class TraceScopedContext {
+ public:
+  TraceScopedContext(const char* category_group,
+                     const char* name,
+                     TraceContext context)
+      : category_group_(category_group), name_(name), context_(context) {
+    TRACE_EVENT_ENTER_CONTEXT(category_group_, name_, context_);
+  }
+
+  ~TraceScopedContext() {
+    TRACE_EVENT_LEAVE_CONTEXT(category_group_, name_, context_);
+  }
+
+ private:
+  const char* category_group_;
+  const char* name_;
+  TraceContext context_;
+  DISALLOW_COPY_AND_ASSIGN(TraceScopedContext);
+};
+
 }  // namespace trace_event_internal
 
 namespace base {
 namespace trace_event {
 
+using trace_event_internal::TraceContext;
+
 template<typename IDType> class TraceScopedTrackableObject {
  public:
   TraceScopedTrackableObject(const char* category_group, const char* name,