Introduce v8.execute tracing category with RunMicrotasks event.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 10835256ccc9d2e021de96cd5a9c8917c7f911ed..7d21090ed2e9fecb8b30aa2a37b5b354e95a3d9f 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2957,17 +2957,21 @@ void Isolate::EnqueueMicrotask(Handle<Object> microtask) {
 
 
 void Isolate::RunMicrotasks() {
-  // Increase call depth to prevent recursive callbacks.
-  v8::Isolate::SuppressMicrotaskExecutionScope suppress(
-      reinterpret_cast<v8::Isolate*>(this));
-  is_running_microtasks_ = true;
-  RunMicrotasksInternal();
-  is_running_microtasks_ = false;
+  if (pending_microtask_count()) {
+    TRACE_EVENT0("v8.execute", "RunMicrotasks");

[caseq, 2016/08/30 16:16:14]

let's move down to RunMicrotasksInternal?

[alph, 2016/08/30 17:06:04]

Done.
Also I noticed that originally FireMicrotasksCompletedCallback were guarded
inside SuppressMicrotaskExecutionScope. Not sure if it's important, but moved
the code to keep the logic.

+    // Increase call depth to prevent recursive callbacks.
+    v8::Isolate::SuppressMicrotaskExecutionScope suppress(
+        reinterpret_cast<v8::Isolate*>(this));
+    is_running_microtasks_ = true;
+    RunMicrotasksInternal();
+    is_running_microtasks_ = false;
+  }
   FireMicrotasksCompletedCallback();
 }
 
 
 void Isolate::RunMicrotasksInternal() {
+  DCHECK(pending_microtask_count() > 0);
   while (pending_microtask_count() > 0) {
     HandleScope scope(this);
     int num_tasks = pending_microtask_count();