Relax default kCanClearUnusedSlotsWithMemset condition

This CL marks non-tracable trivially_destructible objects as
kCanClearUnusedSlotsWithMemset, so that such a class can be stored in
a HeapVector.

Change-Id: Ia4c7a0d0fe7d089d8e9b4446366699ae81554842
Reviewed-on: https://chromium-review.googlesource.com/896763Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533629}

third_party/WebKit/Source/platform/wtf/VectorTraits.h

@@ -42,9 +42,9 @@ struct VectorTraitsBase {
   // slot in the vector's backing storage; it does not have to be equal to
   // what its constructor(s) would create, only be valid for those two uses.
   static constexpr bool kCanClearUnusedSlotsWithMemset =
-      std::is_trivially_constructible<T>::value &&
       std::is_trivially_destructible<T>::value &&
-      std::is_trivially_copyable<T>::value;
+      (!IsTraceable<T>::value || (std::is_trivially_constructible<T>::value &&
+                                  std::is_trivially_copyable<T>::value));
 
   static constexpr bool kCanMoveWithMemcpy =
       std::is_trivially_move_assignable<T>::value;