Introduce v8.execute tracing category with RunMicrotasks event.

The event is used by DevTools to mark microtask execution intervals.
To reduces the overhead the event is only emitted when there are
microtasks to run.

BUG=642228

Review-Url: https://codereview.chromium.org/2289593005
Cr-Commit-Position: refs/heads/master@{#39055}

src/isolate.cc

@@ -2952,6 +2952,8 @@ void Isolate::RunMicrotasks() {
 
 
 void Isolate::RunMicrotasksInternal() {
+  if (!pending_microtask_count()) return;
+  TRACE_EVENT0("v8.execute", "RunMicrotasks");
   while (pending_microtask_count() > 0) {
     HandleScope scope(this);
     int num_tasks = pending_microtask_count();