Create a microtask scope before performing a microtask checkpoint

When the embedder uses microtask scopes to control microtask execution,
we check in the V8 API that there's always a microtask scope on the
stack when entering V8. However, a microtask might call into the
embedder which in turn can call into V8, so we have to put a fake scope
on the stack before executing microtasks to avoid hitting the check.

Bug: chromium:728583
Change-Id: I99bcbcca26eaaac827d0ec09064044f616fb9efc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2853597
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74248}

src/execution/microtask-queue.cc

@@ -113,6 +113,16 @@ void MicrotaskQueue::EnqueueMicrotask(Microtask microtask) {
 void MicrotaskQueue::PerformCheckpoint(v8::Isolate* v8_isolate) {
   if (!IsRunningMicrotasks() && !GetMicrotasksScopeDepth() &&
       !HasMicrotasksSuppressions()) {
+    std::unique_ptr<MicrotasksScope> microtasks_scope;
+    if (microtasks_policy_ == v8::MicrotasksPolicy::kScoped) {
+      // If we're using microtask scopes to schedule microtask execution, V8
+      // API calls will check that there's always a microtask scope on the
+      // stack. As the microtasks we're about to execute could invoke embedder
+      // callbacks which then calls back into V8, we create an artificial
+      // microtask scope here to avoid running into the CallDepthScope check.
+      microtasks_scope.reset(new v8::MicrotasksScope(
+          v8_isolate, this, v8::MicrotasksScope::kDoNotRunMicrotasks));
+    }
     Isolate* isolate = reinterpret_cast<Isolate*>(v8_isolate);
     RunMicrotasks(isolate);
     isolate->ClearKeptObjects();