Make SkImageFilter::Cache purging methods pure virtual

tests/ImageFilterCacheTest.cpp

Index: tests/ImageFilterCacheTest.cpp
diff --git a/tests/ImageFilterCacheTest.cpp b/tests/ImageFilterCacheTest.cpp
index 1337d016b755d3457f84f3b7d59a3491ccbd1f43..688069ea45908a8fb6958515ff0936aa51bb25ea 100644
--- a/tests/ImageFilterCacheTest.cpp
+++ b/tests/ImageFilterCacheTest.cpp
@@ -113,6 +113,7 @@ static void test_explicit_purging(skiatest::Reporter* reporter,
     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;
 
@@ -120,11 +121,13 @@ static void test_explicit_purging(skiatest::Reporter* reporter,
     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));