Introduce --job-based-sweeping flag and use individual jobs for sweeping if set

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 0773d0266683eb712f101a8f1bb026f3ca8d838a..83d619937050274b08ef44585e507a26584f9507 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -744,6 +744,8 @@ class MarkCompactCollector {
   void MarkAllocationSite(AllocationSite* site);
 
  private:
+  class SweeperTask;
+
   explicit MarkCompactCollector(Heap* heap);
   ~MarkCompactCollector();
 
@@ -791,6 +793,8 @@ class MarkCompactCollector {
   // True if concurrent or parallel sweeping is currently in progress.
   bool sweeping_pending_;
 
+  Semaphore pending_sweeper_jobs_semaphore_;
+
   bool sequential_sweeping_;
 
   // A pointer to the current stack-allocated GC tracer object during a full