SkImage_Raster's pixels are always immutable.

src/image/SkSurface_Base.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 SkSurface_Base_DEFINED
 #define SkSurface_Base_DEFINED
 
@@ skipping 50 matching lines @@
      */
     virtual void onDiscard() {}
 
     /**
      *  If the surface is about to change, we call this so that our subclass
      *  can optionally fork their backend (copy-on-write) in case it was
      *  being shared with the cachedImage.
      */
     virtual void onCopyOnWrite(ContentChangeMode) = 0;
 
+    /**
+     *  Signal the surface to remind its backing store that it's mutable again.
+     *  Called only when we _didn't_ copy-on-write; we assume the copies start mutable.
+     */
+    virtual void onRestoreBackingMutability() {}
+
     inline SkCanvas* getCachedCanvas();
     inline SkImage* getCachedImage(Budgeted);
 
+    bool hasCachedImage() const { return fCachedImage != NULL; }
+
     // called by SkSurface to compute a new genID
     uint32_t newGenerationID();
 
 private:
     SkCanvas*   fCachedCanvas;
     SkImage*    fCachedImage;
 
     void aboutToDraw(ContentChangeMode mode);
 
     // Returns true if there is an outstanding image-snapshot, indicating that a call to aboutToDraw
@@ skipping 18 matching lines @@
 
 SkImage* SkSurface_Base::getCachedImage(Budgeted budgeted) {
     if (NULL == fCachedImage) {
         fCachedImage = this->onNewImageSnapshot(budgeted);
         SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
     }
     return fCachedImage;
 }
 
 #endif