[unified-heap] Fix termination sequence for EmbedderHeapTracer

Garbage collection needs to be finalized before unsetting the
EmbedderHeapTracer pointer.

Otherwise, an already running GC may not be properly finalized and the
GC is left in a undefined state which the subsequent follow up GCs
discover.

Bug: 843903
Change-Id: I4dc73005174a1b99dfda96ef379fe62bf5b34dbe
Reviewed-on: https://chromium-review.googlesource.com/1243247Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594024}

third_party/blink/renderer/platform/bindings/v8_per_isolate_data.cc

@@ -162,6 +162,13 @@ void V8PerIsolateData::WillBeDestroyed(v8::Isolate* isolate) {
   data->active_script_wrappables_.Clear();
 
   // Detach V8's garbage collector.
+  if (RuntimeEnabledFeatures::HeapUnifiedGarbageCollectionEnabled()) {
+    // Need to finalize an already running garbage collection as otherwise
+    // callbacks are missing and state gets out of sync.
+    ThreadState::Current()->FinishIncrementalMarkingIfRunning(
+        BlinkGC::kHeapPointersOnStack, BlinkGC::kAtomicMarking,
+        BlinkGC::kEagerSweeping, BlinkGC::GCReason::kThreadTerminationGC);
+  }
   isolate->SetEmbedderHeapTracer(nullptr);
   if (data->script_wrappable_visitor_->WrapperTracingInProgress())
     data->script_wrappable_visitor_->AbortTracing();