scaling API on SkPixmap

BUG=skia:4481

Review URL: https://codereview.chromium.org/1463373002
diff --git a/src/core/SkBitmapProvider.h b/src/core/SkBitmapProvider.h
index 72964e4..9901c0f 100644
--- a/src/core/SkBitmapProvider.h
+++ b/src/core/SkBitmapProvider.h
@@ -27,6 +27,7 @@
 
     bool validForDrawing() const;
     SkImageInfo info() const;
+    bool isVolatile() const;
 
     SkBitmapCacheDesc makeCacheDesc(int w, int h) const;
     SkBitmapCacheDesc makeCacheDesc() const;