[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

Review URL: https://codereview.chromium.org/1470253002

Cr-Commit-Position: refs/heads/master@{#32349}

src/heap/mark-compact.h

@@ -327,6 +327,15 @@ class ThreadLocalTop;
 // Mark-Compact collector
 class MarkCompactCollector {
  public:
+  enum IterationMode {
+    kKeepMarking,
+    kClearMarkbits,
+  };
+
+  class EvacuateNewSpaceVisitor;
+  class EvacuateOldSpaceVisitor;
+  class HeapObjectVisitor;
+
   static void Initialize();
 
   void SetUp();
@@ -703,14 +712,13 @@ 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);