heap: Don't Trace UntracedMember concurrently

Now that UntracedMember don't use atomic writes anymore, it is no longer
safe to concurrently trace collections holding UntracedMember.
Since UntracedMember is not traceable, this can only affect in practice
HashMap where the key is an UntracedMember (such HashMaps could have
been concurrently traced so far and won't anymore). According to code
search, no such HashMaps currently exist, so this CL should not affect
performance.

Bug: 986235
Change-Id: Id4eb925cb5bd2001a8d8e845c746e6da5fd76925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353455
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798118}

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

@@ -774,8 +774,6 @@ struct VectorTraits<blink::UntracedMember<T>>
   static const bool kCanInitializeWithMemset = true;
   static const bool kCanClearUnusedSlotsWithMemset = true;
   static const bool kCanMoveWithMemcpy = true;
-
-  static constexpr bool kCanTraceConcurrently = true;
 };
 
 template <typename T>
@@ -908,8 +906,6 @@ struct HashTraits<blink::UntracedMember<T>>
   static PeekOutType Peek(const blink::UntracedMember<T>& value) {
     return value;
   }
-
-  static constexpr bool kCanTraceConcurrently = true;
 };
 
 template <typename T, wtf_size_t inlineCapacity>