Add a transient image filter cache to SkImage::makeWithFilter & PDF

src/core/SkImageFilterCache.h

Index: src/core/SkImageFilterCache.h
diff --git a/src/core/SkImageFilterCache.h b/src/core/SkImageFilterCache.h
index 25079e194fc6b5d61ce05e64bf6ca6aba5ddc225..a65357f69cb03899daebc949ce2f0630126a4f45 100644
--- a/src/core/SkImageFilterCache.h
+++ b/src/core/SkImageFilterCache.h
@@ -48,6 +48,8 @@ struct SkImageFilterCacheKey {
 // (result, offset).
 class SkImageFilterCache : public SkRefCnt {
 public:
+    enum { kDefaultTransientSize = 32 * 1024 * 1024 };
+
     virtual ~SkImageFilterCache() {}
     static SkImageFilterCache* Create(size_t maxBytes);
     static SkImageFilterCache* Get();