[heap] Optimize young generation marking

- Only mark a single bit (grey)
- Increment live bytes after visiting, avoiding the map lookup for size
  in ObjectMarking

Raw speed improvements should be around 20%-30%.

Bug: chromium:651354
Change-Id: Ib58d1aee0b99d8e628a0191f90a2ffad9324b915
Reviewed-on: https://chromium-review.googlesource.com/509548
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45484}

src/heap/incremental-marking.cc

@@ -835,16 +835,15 @@ void IncrementalMarking::UpdateMarkingDequeAfterScavenge() {
       // The object may be on a page that was moved in new space.
       DCHECK(
           Page::FromAddress(obj->address())->IsFlagSet(Page::SWEEP_TO_ITERATE));
-      return ObjectMarking::IsBlack<kAtomicity>(obj,
-                                                MarkingState::External(obj))
+      return ObjectMarking::IsGrey<kAtomicity>(obj, MarkingState::External(obj))
                  ? obj
                  : nullptr;
     } else {
       // The object may be on a page that was moved from new to old space.
       if (Page::FromAddress(obj->address())
               ->IsFlagSet(Page::SWEEP_TO_ITERATE)) {
-        return ObjectMarking::IsBlack<kAtomicity>(obj,
-                                                  MarkingState::External(obj))
+        return ObjectMarking::IsGrey<kAtomicity>(obj,
+                                                 MarkingState::External(obj))
                    ? obj
                    : nullptr;
       }

src/heap/mark-compact.h

@@ -218,6 +218,12 @@ class LiveObjectVisitor BASE_EMBEDDED {
   bool VisitBlackObjects(MemoryChunk* chunk, const MarkingState& state,
                          Visitor* visitor, IterationMode iteration_mode);
 
+  // Visits grey objects on a Memorychunk. Is not allowed to fail visitation
+  // for an object.
+  template <class Visitor>
+  bool VisitGreyObjectsNoFail(MemoryChunk* chunk, const MarkingState& state,
+                              Visitor* visitor, IterationMode iteration_mode);
+
  private:
   void RecomputeLiveBytes(MemoryChunk* chunk, const MarkingState& state);
 };