add preroll to image, for chrome's warmup pass

include/core/SkImage.h

Index: include/core/SkImage.h
diff --git a/include/core/SkImage.h b/include/core/SkImage.h
index 5b9e2031758f8ebc317041289a0603f8113d1ece..2bcb90b5d3bebec7adb0b79d5dc4dde4c108f7c2 100644
--- a/include/core/SkImage.h
+++ b/include/core/SkImage.h
@@ -176,6 +176,20 @@ public:
      */
     bool peekPixels(SkPixmap* pixmap) const;
 
+    /**
+     *  Some images have to perform preliminary work in preparation for drawing. This can be
+     *  decoding, uploading to a GPU, or other tasks. These happen automatically when an image
+     *  is drawn, and often they are cached so that the cost is only paid the first time.
+     *
+     *  Preroll() can be called before drawing to try to perform this prepatory work ahead of time.
+     *  For images that have no such work, this returns instantly. Others may do some thing to
+     *  prepare their cache and then return.
+     *
+     *  If the image will drawn to a GPU-backed canvas or surface, pass the associated GrContext.
+     *  If the image will be drawn to any other type of canvas or surface, pass null.
+     */
+    void preroll(GrContext* = nullptr) const;
+
     // DEPRECATED
     GrTexture* getTexture() const;