Avoid nested RunMicrotasks on d8

%RunMicrotasks() call used to cause a nested RunMicrotasks if it's
called from a microtask, and that breaks its invariants.
This CL fixes it by disabling the nested loop.

Change-Id: I2f4a81be0dbf0554abf6eb41b62c25465338f29a
Reviewed-on: https://chromium-review.googlesource.com/c/1358036Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58026}

src/d8.cc

@@ -3013,7 +3013,7 @@ bool ProcessMessages(
     SealHandleScope shs(isolate);
     while (v8::platform::PumpMessageLoop(g_default_platform, isolate,
                                          behavior())) {
-      isolate->RunMicrotasks();
+      MicrotasksScope::PerformCheckpoint(isolate);
     }
     if (g_default_platform->IdleTasksEnabled(isolate)) {
       v8::platform::RunIdleTasks(g_default_platform, isolate,

src/isolate.cc

@@ -4310,6 +4310,8 @@ void Isolate::EnqueueMicrotask(Handle<Microtask> microtask) {
 
 
 void Isolate::RunMicrotasks() {
+  DCHECK(!is_running_microtasks_);
+
   // TODO(tzik): Move the suppression, |is_running_microtask_|, and the
   // completion callbacks into MicrotaskQueue.