[heap] Set class RememberedSet to AllStatic

Since class RememberedSet only contains static methods, it'd be better
to make it as AllStatic class.

BUG=
R=ulan@chromium.org, yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2570783004
Cr-Commit-Position: refs/heads/master@{#41678}

src/heap/remembered-set.h

@@ -17,7 +17,7 @@ enum PointerDirection { OLD_TO_OLD, OLD_TO_NEW };
 
 // TODO(ulan): Investigate performance of de-templatizing this class.
 template <PointerDirection direction>
-class RememberedSet {
+class RememberedSet : public AllStatic {
  public:
   // Given a page and a slot in that page, this function adds the slot to the
   // remembered set.