[heap] Do not clear mark bits of black large objects on right trim.

BUG=

Review-Url: https://codereview.chromium.org/2592603002
Cr-Commit-Position: refs/heads/master@{#41838}

src/heap/heap.cc

@@ -3234,8 +3234,6 @@ void Heap::RightTrimFixedArray(FixedArrayBase* object, int elements_to_trim) {
   // of the object changed significantly.
   if (!lo_space()->Contains(object)) {
     CreateFillerObjectAt(new_end, bytes_to_trim, ClearRecordedSlots::kYes);
-  }
-
     // 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.
     if (incremental_marking()->black_allocation() &&
@@ -3245,6 +3243,7 @@ void Heap::RightTrimFixedArray(FixedArrayBase* object, int elements_to_trim) {
           page->AddressToMarkbitIndex(new_end),
           page->AddressToMarkbitIndex(new_end + bytes_to_trim));
     }
+  }
 
   // Initialize header of the trimmed array. We are storing the new length
   // using release store after creating a filler for the left-over space to