heap: Fix race in ephemeron tracing.

Ephemeron tracing used to be only on mutator thread. We recently
restructured it in a way that allows for concurrenncy. This, the
TraceEphemeron method can be concurrent and should use GetSafe.

Bug: 986235
Change-Id: Ib10ad7eef383f41d2a70d966d04904a54bc96e76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358970
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799175}

third_party/blink/renderer/platform/heap/visitor.h

@@ -200,7 +200,7 @@ class PLATFORM_EXPORT Visitor {
   void TraceEphemeron(const WeakMember<T>& key,
                       const void* value,
                       TraceCallback value_trace_callback) {
-    T* t = key.Get();
+    const T* t = key.GetSafe();
     if (!t)
       return;
     VisitEphemeron(TraceDescriptorFor(t).base_object_payload, value,