Make SkImageFilter::Cache purging methods pure virtual

src/core/SkImageFilter.cpp

 /*
  * Copyright 2012 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkImageFilter.h"
 #include "SkImageFilterCacheKey.h"
 
@@ skipping 628 matching lines @@
 
     void purgeByKeys(const Key keys[], int count) override {
         SkAutoMutexAcquire mutex(fMutex);
         for (int i = 0; i < count; i++) {
             if (Value* v = fLookup.find(keys[i])) {
                 this->removeInternal(v);
             }
         }
     }
 
+    SkDEBUGCODE(int count() const override { return fLookup.count(); })
 private:
     void removeInternal(Value* v) {
         if (v->fImage) {
             fCurrentBytes -= v->fImage->getSize();
         } else {
             fCurrentBytes -= v->fBitmap.getSize();
         }
         fLRU.remove(v);
         fLookup.remove(v->fKey);
         delete v;
@@ skipping 31 matching lines @@
                                    false,   /* preserveLCDText */
                                    true /*forImageFilter*/);
     SkBaseDevice* dev = fDevice->onCreateDevice(cinfo, nullptr);
     if (nullptr == dev) {
         const SkSurfaceProps surfaceProps(fDevice->fSurfaceProps.flags(),
                                           kUnknown_SkPixelGeometry);
         dev = SkBitmapDevice::Create(cinfo.fInfo, surfaceProps);
     }
     return dev;
 }