Make SkImageFilter::Cache purging methods pure virtual

tests/ImageFilterCacheTest.cpp

  /*
   * Copyright 2016 Google Inc.
   *
   * Use of this source code is governed by a BSD-style license that can be
   * found in the LICENSE file.
   */
 
 #include "Test.h"
 
 #include "SkBitmap.h"
@@ skipping 95 matching lines @@
     static const size_t kCacheSize = 1000000;
     SkAutoTUnref<SkImageFilter::Cache> cache(SkImageFilter::Cache::Create(kCacheSize));
 
     SkIRect clip = SkIRect::MakeWH(100, 100);
     SkImageFilter::Cache::Key key1(0, SkMatrix::I(), clip, image->uniqueID(), image->subset());
     SkImageFilter::Cache::Key key2(1, SkMatrix::I(), clip, subset->uniqueID(), image->subset());
 
     SkIPoint offset = SkIPoint::Make(3, 4);
     cache->set(key1, image.get(), offset);
     cache->set(key2, image.get(), offset);
+    SkDEBUGCODE(REPORTER_ASSERT(reporter, 2 == cache->count());)
 
     SkIPoint foundOffset;
 
     REPORTER_ASSERT(reporter, cache->get(key1, &foundOffset));
     REPORTER_ASSERT(reporter, cache->get(key2, &foundOffset));
 
     cache->purgeByKeys(&key1, 1);
+    SkDEBUGCODE(REPORTER_ASSERT(reporter, 1 == cache->count());)
 
     REPORTER_ASSERT(reporter, !cache->get(key1, &foundOffset));
     REPORTER_ASSERT(reporter, cache->get(key2, &foundOffset));
 
     cache->purge();
+    SkDEBUGCODE(REPORTER_ASSERT(reporter, 0 == cache->count());)
 
     REPORTER_ASSERT(reporter, !cache->get(key1, &foundOffset));
     REPORTER_ASSERT(reporter, !cache->get(key2, &foundOffset));
 }
 
 DEF_TEST(ImageFilterCache_RasterBacked, reporter) {
     SkBitmap srcBM = create_bm();
 
     const SkIRect& full = SkIRect::MakeWH(kFullSize, kFullSize);
 
@@ skipping 88 matching lines @@
     sk_sp<SkSpecialImage> subsetImg(SkSpecialImage::MakeFromGpu(nullptr, subset,
                                                                 kNeedNewImageUniqueID_SpecialImage,
                                                                 srcTexture));
 
     test_find_existing(reporter, fullImg, subsetImg);
     test_dont_find_if_diff_key(reporter, fullImg, subsetImg);
     test_internal_purge(reporter, fullImg);
     test_explicit_purging(reporter, fullImg, subsetImg);
 }
 #endif