Reland of "[heap] Add more tasks for parallel compaction"

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 6558eb2ddebba2b6349c64eb2fbf4f2c9ea40247..724650c1c4e01492669f88524cc5ee071cb1b6ae 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -709,11 +709,8 @@ class MarkCompactCollector {
 
   void EvacuatePagesInParallel();
 
-  int NumberOfParallelCompactionTasks() {
-    // TODO(hpayer, mlippautz): Figure out some logic to determine the number
-    // of compaction tasks.
-    return 1;
-  }
+  // The number of parallel compaction tasks, including the main thread.
+  int NumberOfParallelCompactionTasks();
 
   void WaitUntilCompactionCompleted();