heap: Don't check unused slot during Vector::Trace

Vector::Trace used to check that all unused slots are zeroed out.
This is a hidden assumption that the Vector cannot be changed while
it is traced. This assumption will no longer once Vectors are traced
concurrently.

Bug: 986235
Change-Id: I05e1270f92f0542595e2c87c776cbf58a785e8cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015245Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746820}

third_party/blink/renderer/platform/wtf/vector.h

@@ -2017,7 +2017,6 @@ Vector<T, inlineCapacity, Allocator>::Trace(VisitorDispatcher visitor) {
         Allocator::template Trace<T, VectorTraits<T>>(
             visitor, *const_cast<T*>(buffer_entry));
       }
-      CheckUnusedSlots(Buffer() + size(), Buffer() + capacity());
     }
   }
 }