add preroll to image, for chrome's warmup pass

src/image/SkImage_Base.h

Index: src/image/SkImage_Base.h
diff --git a/src/image/SkImage_Base.h b/src/image/SkImage_Base.h
index 2e3fb9a34a81236e42d860b31f8b45edc4c57761..01b39be383abe25299881113ba9e45b074e2659d 100644
--- a/src/image/SkImage_Base.h
+++ b/src/image/SkImage_Base.h
@@ -51,7 +51,9 @@ public:
     // Default impl calls onDraw
     virtual bool onReadPixels(const SkImageInfo& dstInfo, void* dstPixels, size_t dstRowBytes,
                               int srcX, int srcY) const;
-    
+
+    virtual void onPreroll(GrContext*) const {}
+
     virtual GrTexture* getTexture() const { return nullptr; }
 
     // return a read-only copy of the pixels. We promise to not modify them,