Mark SkBBoxHierarchyRecord destructor as virtual

Immediate parent classes have empty virtual destructors, but farther
up the inheritance tree there is complicated destruction going on.
This change may be unnecessary but makes the polymorphism explicit.

BUG=skia:1734
R=mtklein@google.com, tomhudson@google.com, mtklein

Author: tomhudson@chromium.org

Review URL: https://codereview.chromium.org/359773002

src/core/SkBBoxHierarchyRecord.h

@@ -20,6 +20,7 @@ class SkBBoxHierarchyRecord : public SkBBoxRecord, public SkBBoxHierarchyClient
 public:
     /** This will take a ref of h */
     SkBBoxHierarchyRecord(const SkISize& size, uint32_t recordFlags, SkBBoxHierarchy* h);
+    virtual ~SkBBoxHierarchyRecord() { };
 
     virtual void handleBBox(const SkRect& bounds) SK_OVERRIDE;