[heap] MarkingVisitor cleanup

Bug: 
Change-Id: Ib3a27251bd8b02780033302364a0ed63613e1998
Reviewed-on: https://chromium-review.googlesource.com/735142Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48857}

src/heap/mark-compact-inl.h

@@ -230,7 +230,7 @@ void MarkingVisitor<fixed_array_mode, retaining_path_mode,
                     MarkingState>::MarkObject(HeapObject* host,
                                               HeapObject* object) {
   if (marking_state()->WhiteToGrey(object)) {
-    collector_->marking_worklist()->Push(object);
+    marking_worklist()->Push(object);
     if (retaining_path_mode == TraceRetainingPathMode::kEnabled &&
         V8_UNLIKELY(FLAG_track_retaining_path)) {
       heap_->AddRetainer(host, object);

src/heap/mark-compact.h

@@ -1011,8 +1011,8 @@ class MarkingVisitor final
 
   MarkingState* marking_state() { return marking_state_; }
 
-  MarkCompactCollector::MarkingWorklist* marking_worklist() {
-    return this->heap_->incremental_marking()->marking_worklist();
+  MarkCompactCollector::MarkingWorklist* marking_worklist() const {
+    return collector_->marking_worklist();
   }
 
   Heap* const heap_;