add preroll to image, for chrome's warmup pass

include/core/SkImage.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImage_DEFINED
 #define SkImage_DEFINED
 
@@ skipping 158 matching lines @@
 
     /**
      *  If the image has direct access to its pixels (i.e. they are in local
      *  RAM) return the (const) address of those pixels, and if not null, return
      *  true, and if pixmap is not NULL, set it to point into the image.
      *
      *  On failure, return false and ignore the pixmap parameter.
      */
     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;
 
     /**
      *  Returns true if the image is texture backed.
      */
     bool isTextureBacked() const;
 
     /**
      *  Retrieves the backend API handle of the texture. If flushPendingGrContextIO then the
@@ skipping 110 matching lines @@
 
 private:
     const int       fWidth;
     const int       fHeight;
     const uint32_t  fUniqueID;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif