restore clipstack to heap-ptr, so clients can ref it

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 1dbca3a0a286448f25ed0c40a3ff6a8d00ce2f46..853877dba7bf2009375da225463a181f45f80d70 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -1089,7 +1089,7 @@ public:
      *  @return the current clip stack ("list" of individual clip elements)
      */
     const SkClipStack* getClipStack() const {
-        return &fClipStack;
+        return fClipStack;
     }
 
     typedef SkCanvasClipVisitor ClipVisitor;
@@ -1251,7 +1251,7 @@ protected:
 private:
     class MCRec;
 
-    SkClipStack fClipStack;
+    SkAutoTUnref<SkClipStack> fClipStack;
     SkDeque     fMCStack;
     // points to top of stack
     MCRec*      fMCRec;