[heap] Remove spurious DCHECK

We already filter out fillers in ProcessMarkingDeque.

BUG=

Change-Id: Iee0dbfffe6e4eb5c65a1c25ba75c4c69bf15355b
Reviewed-on: https://chromium-review.googlesource.com/453481Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43754}

src/heap/incremental-marking.cc

@@ -796,7 +796,6 @@ void IncrementalMarking::VisitObject(Map* map, HeapObject* obj, int size) {
   MarkBit mark_bit = ObjectMarking::MarkBitFrom(obj);
   MemoryChunk* chunk = MemoryChunk::FromAddress(obj->address());
   SLOW_DCHECK(Marking::IsGrey(mark_bit) ||
-              (obj->IsFiller() && Marking::IsWhite(mark_bit)) ||
               (chunk->IsFlagSet(MemoryChunk::HAS_PROGRESS_BAR) &&
                Marking::IsBlack(mark_bit)));
 #endif