Ensure runtime call stats show up for microtasks

Prior to this, traces recorded through chrome://tracing would not
include time spent in RunMicrotasks.

BUG=v8:5382

Review-Url: https://codereview.chromium.org/2592793003
Cr-Commit-Position: refs/heads/master@{#42316}

src/isolate.cc

@@ -3385,6 +3385,7 @@ void Isolate::RunMicrotasks() {
 void Isolate::RunMicrotasksInternal() {
   if (!pending_microtask_count()) return;
   TRACE_EVENT0("v8.execute", "RunMicrotasks");
+  TRACE_EVENT_CALL_STATS_SCOPED(this, "v8", "V8.RunMicrotasks");
   while (pending_microtask_count() > 0) {
     HandleScope scope(this);
     int num_tasks = pending_microtask_count();