Just clear semi-space mark bits once before Scavenge

Both PrepareForScavenge and ResetAllocationInfo clear semi-space mark bits before Scavenge. Doing this once is good enough.

BUG=

Review URL: https://codereview.chromium.org/1158623003

Cr-Commit-Position: refs/heads/master@{#28778}

src/heap/heap.cc

@@ -1595,8 +1595,6 @@ void Heap::Scavenge() {
 
   SelectScavengingVisitorsTable();
 
-  incremental_marking()->PrepareForScavenge();
-
   // Flip the semispaces.  After flipping, to space is empty, from space has
   // live objects.
   new_space_.Flip();

src/heap/incremental-marking.cc

@@ -578,16 +578,6 @@ void IncrementalMarking::MarkObjectGroups() {
 }
 
 
-void IncrementalMarking::PrepareForScavenge() {
-  if (!IsMarking()) return;
-  NewSpacePageIterator it(heap_->new_space()->FromSpaceStart(),
-                          heap_->new_space()->FromSpaceEnd());
-  while (it.has_next()) {
-    Bitmap::Clear(it.next());
-  }
-}
-
-
 void IncrementalMarking::UpdateMarkingDequeAfterScavenge() {
   if (!IsMarking()) return;
 

src/heap/incremental-marking.h

@@ -73,8 +73,6 @@ class IncrementalMarking {
 
   void MarkObjectGroups();
 
-  void PrepareForScavenge();
-
   void UpdateMarkingDequeAfterScavenge();
 
   void Hurry();