Implement SkImageFilter::Cache with SkResourceCache.

include/core/SkImageFilter.h

Index: include/core/SkImageFilter.h
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index 1ca1c01c08b2b4c49827358b0d52c2fa8a2dd35d..948415993631b1606c187938e6f31edcfc5c03f3 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -42,7 +42,6 @@ public:
         static Cache* Get();
         virtual bool get(const Key& key, SkBitmap* result, SkIPoint* offset) const = 0;
         virtual void set(const Key& key, const SkBitmap& result, const SkIPoint& offset) = 0;
-        virtual void purge() {}
     };
 
     class Context {
@@ -375,9 +374,6 @@ protected:
     virtual bool affectsTransparentBlack() const;
 
 private:
-    friend class SkGraphics;
-    static void PurgeCache();
-
     bool usesSrcInput() const { return fUsesSrcInput; }
 
     typedef SkFlattenable INHERITED;