[unified-heap] Adjust ScriptPromiseProperty GC test

The test checks that the JS wrapper gets collected while the C++
wrappable stays alive. This only worked by accident as wrapper tracing
does not see any stack pointers.

Adjust test to check that the JS wrapper gets collected as soon as the
C++ wrappable is cleared.

Bug: 843903
Change-Id: I2041f682f4a94d690f43292f70ea3e8d7fb3fd6b
Reviewed-on: https://chromium-review.googlesource.com/c/1293450Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601554}

third_party/blink/renderer/bindings/core/v8/script_promise_property_test.cc

@@ -130,7 +130,9 @@ class ScriptPromisePropertyTestBase {
   }
 
   void Gc() {
-    V8GCController::CollectAllGarbageForTesting(v8::Isolate::GetCurrent());
+    V8GCController::CollectAllGarbageForTesting(
+        v8::Isolate::GetCurrent(),
+        v8::EmbedderHeapTracer::EmbedderStackState::kEmpty);
   }
 
   v8::Local<v8::Function> NotReached(ScriptState* script_state) {
@@ -170,6 +172,7 @@ class ScriptPromisePropertyGarbageCollectedTest
   ScriptPromisePropertyGarbageCollectedTest()
       : holder_(new GarbageCollectedHolder(&GetDocument())) {}
 
+  void ClearHolder() { holder_.Clear(); }
   GarbageCollectedHolder* Holder() { return holder_; }
   Property* GetProperty() { return holder_->GetProperty(); }
   ScriptPromise Promise(DOMWrapperWorld& world) {
@@ -290,10 +293,9 @@ TEST_F(ScriptPromisePropertyGarbageCollectedTest,
   EXPECT_FALSE(observation->wasCollected());
 
   holder_wrapper.Clear();
+  ClearHolder();
   Gc();
   EXPECT_TRUE(observation->wasCollected());
-
-  EXPECT_EQ(Property::kPending, GetProperty()->GetState());
 }
 
 TEST_F(ScriptPromisePropertyGarbageCollectedTest,