Reland: Add a trace-event for each runtime-stats timer (CL 2052523002)

src/tracing/trace-event.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/tracing/trace-event.h"
 
 #include "src/v8.h"
 
+// A global flag used as a shortcut to check for the
+// v8.runtime category due to its high frequency use.
+TRACE_EVENT_API_ATOMIC_BYTE g_runtime_calls_trace_enabled = 0;
+
 namespace v8 {
 namespace internal {
 namespace tracing {
 
 v8::Platform* TraceEventHelper::GetCurrentPlatform() {
   return v8::internal::V8::GetCurrentPlatform();
 }
 
 }  // namespace tracing
 }  // namespace internal
 }  // namespace v8