Create a hash table from id<-->key in SkImageFilter::CacheImpl

include/core/SkImageFilter.h

Index: include/core/SkImageFilter.h
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index 865fcf8b97b1310e551d35add15f4d9d77d93543..6ece16c84101d03efc37a3e6b78a18cc6a0de662 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -42,6 +42,7 @@ public:
         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() {}
+        virtual void purgeByImageFilterId(uint32_t) {}
     };
 
     enum SizeConstraint {