Add basic support for parallel compaction and flag.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 843e73d8e7dfa7d026036e70a8c80d9cd206204f..12da2d66747a939140dcdaf0816574513cd9ad03 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -668,6 +668,7 @@ class MarkCompactCollector {
   void RemoveObjectSlots(Address start_slot, Address end_slot);
 
  private:
+  class CompactionTask;
   class SweeperTask;
 
   explicit MarkCompactCollector(Heap* heap);
@@ -705,8 +706,12 @@ class MarkCompactCollector {
   // True if concurrent or parallel sweeping is currently in progress.
   bool sweeping_in_progress_;
 
+  // Synchronize sweeper threads.
   base::Semaphore pending_sweeper_jobs_semaphore_;
 
+  // Synchronize compaction threads.
+  base::Semaphore pending_compaction_jobs_semaphore_;
+
   bool evacuation_;
 
   SlotsBufferAllocator slots_buffer_allocator_;
@@ -865,6 +870,8 @@ class MarkCompactCollector {
 
   void EvacuatePages();
 
+  void EvacuatePagesInParallel();
+
   void EvacuateNewSpaceAndCandidates();
 
   void ReleaseEvacuationCandidates();