Make SkImageFilter::Cache purging methods pure virtual

include/core/SkImageFilter.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImageFilter_DEFINED
 #define SkImageFilter_DEFINED
 
@@ skipping 29 matching lines @@
     class Cache : public SkRefCnt {
     public:
         struct Key;
         virtual ~Cache() {}
         static Cache* Create(size_t maxBytes);
         static Cache* Get();
         virtual bool get(const Key& key, SkBitmap* result, SkIPoint* offset) const = 0;
         virtual SkSpecialImage* get(const Key& key, SkIPoint* offset) const = 0;
         virtual void set(const Key& key, const SkBitmap& result, const SkIPoint& offset) = 0;
         virtual void set(const Key& key, SkSpecialImage* image, const SkIPoint& offset) = 0;
-        virtual void purge() {}
-        virtual void purgeByKeys(const Key[], int) {}
+        virtual void purge() = 0;
+        virtual void purgeByKeys(const Key[], int) = 0;
+        SkDEBUGCODE(virtual int count() const = 0;)
     };
 
     class Context {
     public:
         Context(const SkMatrix& ctm, const SkIRect& clipBounds, Cache* cache)
             : fCTM(ctm)
             , fClipBounds(clipBounds)
             , fCache(cache)
         {}
 
@@ skipping 367 matching lines @@
  */
 #define SK_IMAGEFILTER_UNFLATTEN_COMMON(localVar, expectedCount)    \
     Common localVar;                                                \
     do {                                                            \
         if (!localVar.unflatten(buffer, expectedCount)) {           \
             return NULL;                                            \
         }                                                           \
     } while (0)
 
 #endif