[heap, runtime] Avoid redundant clearing of slots in a trimmed array.

The slots outside the trimmed array will be cleared by the sweeper and
will not be overwritten with an untagged value.

BUG=chromium:694255

Change-Id: I3e814b9934ca95a09e883e237687434e6bb58c80
Reviewed-on: https://chromium-review.googlesource.com/591651Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46998}

src/heap/heap.cc

@@ -3404,7 +3404,8 @@ FixedArrayBase* Heap::LeftTrimFixedArray(FixedArrayBase* object,
   // Technically in new space this write might be omitted (except for
   // debug mode which iterates through the heap), but to play safer
   // we still do it.
-  CreateFillerObjectAt(old_start, bytes_to_trim, ClearRecordedSlots::kYes);
+  // Recorded slots will be cleared by the sweeper.
+  CreateFillerObjectAt(old_start, bytes_to_trim, ClearRecordedSlots::kNo);
 
   // Initialize header of the trimmed array. Since left trimming is only
   // performed on pages which are not concurrently swept creating a filler
@@ -3473,8 +3474,9 @@ void Heap::RightTrimFixedArray(FixedArrayBase* object, int elements_to_trim) {
   // TODO(hpayer): We should shrink the large object page if the size
   // of the object changed significantly.
   if (!lo_space()->Contains(object)) {
+    // Recorded slots will be cleared by the sweeper.
     HeapObject* filler =
-        CreateFillerObjectAt(new_end, bytes_to_trim, ClearRecordedSlots::kYes);
+        CreateFillerObjectAt(new_end, bytes_to_trim, ClearRecordedSlots::kNo);
     DCHECK_NOT_NULL(filler);
     // Clear the mark bits of the black area that belongs now to the filler.
     // This is an optimization. The sweeper will release black fillers anyway.