[api] Add Context::GetMicrotaskQueue method

Add a method that returns the microtask queue that is being used
by the `v8::Context`.

This is helpful in non-monolithic embedders like Node.js, which
accept Contexts created by its own embedders like Electron, or
for native Node.js addons. In particular, it enables:

1. Making sure that “nested” `Context`s use the correct microtask
   queue, i.e. the one from the outer Context.
2. Enqueueing microtasks into the correct microtask queue.

Previously, these things only worked when the microtask queue for
a given Context was the Isolate’s default queue.

As an alternative, I considered adding a way to make new `Context`s
inherit the queue from the `Context` that was entered at the time
of their creation, but that seemed a bit more “magic”, less flexible,
and didn’t take care of concern 2 listed above.

Change-Id: I15ed796df90f23c97a545a8e1b30a3bf4a5c4320
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2579914Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71710}

include/v8.h

@@ -10554,9 +10554,12 @@ class V8_EXPORT Context {
    */
   void Exit();
 
-  /** Returns an isolate associated with a current context. */
+  /** Returns the isolate associated with a current context. */
   Isolate* GetIsolate();
 
+  /** Returns the microtask queue associated with a current context. */
+  MicrotaskQueue* GetMicrotaskQueue();
+
   /**
    * The field at kDebugIdIndex used to be reserved for the inspector.
    * It now serves no purpose.

src/api/api.cc

@@ -6261,6 +6261,12 @@ v8::Isolate* Context::GetIsolate() {
   return reinterpret_cast<Isolate*>(env->GetIsolate());
 }
 
+v8::MicrotaskQueue* Context::GetMicrotaskQueue() {
+  i::Handle<i::Context> env = Utils::OpenHandle(this);
+  CHECK(env->IsNativeContext());
+  return i::Handle<i::NativeContext>::cast(env)->microtask_queue();
+}
+
 v8::Local<v8::Object> Context::Global() {
   i::Handle<i::Context> context = Utils::OpenHandle(this);
   i::Isolate* isolate = context->GetIsolate();

test/cctest/test-api.cc

@@ -28801,3 +28801,13 @@ UNINITIALIZED_TEST(SingleThreadedDefaultPlatform) {
   isolate->Dispose();
   i::V8::SetPlatformForTesting(old_platform);
 }
+
+THREADED_TEST(MicrotaskQueueOfContext) {
+  auto microtask_queue = v8::MicrotaskQueue::New(CcTest::isolate());
+  v8::HandleScope scope(CcTest::isolate());
+  v8::Local<Context> context = Context::New(
+      CcTest::isolate(), nullptr, v8::MaybeLocal<ObjectTemplate>(),
+      v8::MaybeLocal<Value>(), v8::DeserializeInternalFieldsCallback(),
+      microtask_queue.get());
+  CHECK_EQ(context->GetMicrotaskQueue(), microtask_queue.get());
+}