Reland of [heap] Refactor evacuation for young and old gen into visitors....

Reland of [heap] Refactor evacuation for young and old gen into visitors. (patchset #1 id:1 of https://codereview.chromium.org/1483393002/ )

Reason for revert:
Reland after fixing the potential root cause of the canary crasher.

Original issue's description:
> Revert of [heap] Refactor evacuation for young and old gen into visitors. (patchset #5 id:80001 of https://codereview.chromium.org/1470253002/ )
>
> Reason for revert:
> Still investigating bad canary.
>
> Original issue's description:
> > [heap] Refactor evacuation for young and old gen into visitors.
> >
> > Create a visitor for evacuating objects for young and old generation. This is
> > the first step of preparing a task to process, both,  newspace and oldspace
> > pages in parallel.
> >
> > BUG=chromium:524425
> > LOG=N
> >
> > Committed: https://crrev.com/138d9bae5d7014e0d205634a49b5eac3697744c8
> > Cr-Commit-Position: refs/heads/master@{#32349}
>
> TBR=mlippautz@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=chromium:524425
>
> Committed: https://crrev.com/aa24a3135ec308e1f84bce334844caf0cae2437a
> Cr-Commit-Position: refs/heads/master@{#32462}

TBR=mlippautz@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:524425

Review URL: https://codereview.chromium.org/1493523003

Cr-Commit-Position: refs/heads/master@{#32500}

src/heap/mark-compact.h

@@ -320,6 +320,15 @@ class ThreadLocalTop;
 // Mark-Compact collector
 class MarkCompactCollector {
  public:
+  enum IterationMode {
+    kKeepMarking,
+    kClearMarkbits,
+  };
+
+  class EvacuateNewSpaceVisitor;
+  class EvacuateOldSpaceVisitor;
+  class HeapObjectVisitor;
+
   static void Initialize();
 
   void SetUp();
@@ -698,14 +707,13 @@ class MarkCompactCollector {
   // regions to each space's free list.
   void SweepSpaces();
 
-  int DiscoverAndEvacuateBlackObjectsOnPage(NewSpace* new_space,
-                                            NewSpacePage* p);
+  // Iterates through all live objects on a page using marking information.
+  // Returns whether all objects have successfully been visited.
+  bool IterateLiveObjectsOnPage(MemoryChunk* page, HeapObjectVisitor* visitor,
+                                IterationMode mode);
 
   void EvacuateNewSpace();
 
-  bool EvacuateLiveObjectsFromPage(Page* p, PagedSpace* target_space,
-                                   SlotsBuffer** evacuation_slots_buffer);
-
   void AddEvacuationSlotsBufferSynchronized(
       SlotsBuffer* evacuation_slots_buffer);