Add tracing for major GC in Sky

sky/engine/tonic/dart_gc_controller.cc

 // Copyright 2015 The Chromium 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 "sky/engine/config.h"
 #include "sky/engine/tonic/dart_gc_controller.h"
 
+#include "base/trace_event/trace_event.h"
 #include "dart/runtime/include/dart_api.h"
 #include "sky/engine/tonic/dart_gc_context.h"
 #include "sky/engine/tonic/dart_gc_visitor.h"
 #include "sky/engine/tonic/dart_wrappable.h"
 
 namespace blink {
 namespace {
 
 DartGCContext* g_gc_context = nullptr;
 
 DartWrappable* GetWrappable(intptr_t* fields) {
   return reinterpret_cast<DartWrappable*>(fields[DartWrappable::kPeerIndex]);
 }
 
 void Visit(void* isolate_callback_data,
            Dart_WeakPersistentHandle handle,
            intptr_t native_field_count,
            intptr_t* native_fields) {
   if (!native_field_count)
     return;
   DCHECK(native_field_count == DartWrappable::kNumberOfNativeFields);
   DartGCVisitor visitor(g_gc_context);
   GetWrappable(native_fields)->AcceptDartGCVisitor(visitor);
 }
 
 }  // namespace
 
 void DartGCPrologue() {
+  TRACE_EVENT_ASYNC_BEGIN0("sky", "DartGC", 0);
+
   Dart_EnterScope();
   DCHECK(!g_gc_context);
   g_gc_context = new DartGCContext();
   Dart_VisitPrologueWeakHandles(Visit);
 }
 
 void DartGCEpilogue() {
   delete g_gc_context;
   g_gc_context = nullptr;
   Dart_ExitScope();
+
+  TRACE_EVENT_ASYNC_END0("sky", "DartGC", 0);
 }
 
 }  // namespace blink