SkImage_Raster's pixels are always immutable.

src/image/SkSurface.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSurface_Base.h"
 #include "SkImagePriv.h"
 #include "SkCanvas.h"
@@ skipping 81 matching lines @@
 
 void SkSurface_Base::aboutToDraw(ContentChangeMode mode) {
     this->dirtyGenerationID();
 
     SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
 
     if (fCachedImage) {
         // the surface may need to fork its backend, if its sharing it with
         // the cached image. Note: we only call if there is an outstanding owner
         // on the image (besides us).
-        if (!fCachedImage->unique()) {
+        bool unique = fCachedImage->unique();
+        if (!unique) {
             this->onCopyOnWrite(mode);
         }
 
         // regardless of copy-on-write, we must drop our cached image now, so
         // that the next request will get our new contents.
         fCachedImage->unref();
         fCachedImage = NULL;
+
+        if (unique) {
+            // Our content isn't held by any image now, so we can consider that content mutable.
+            // Raster surfaces need to be told it's safe to consider its pixels mutable again.
+            // We make this call after the ->unref() so the subclass can assert there are no images.
+            this->onRestoreBackingMutability();
+        }
     } else if (kDiscard_ContentChangeMode == mode) {
         this->onDiscard();
     }
 }
 
 uint32_t SkSurface_Base::newGenerationID() {
     SkASSERT(!fCachedCanvas || fCachedCanvas->getSurfaceBase() == this);
     static int32_t gID;
     return sk_atomic_inc(&gID) + 1;
 }
@@ skipping 84 matching lines @@
                                              const SkSurfaceProps*) {
     return NULL;
 }
 
 SkSurface* SkSurface::NewFromBackendRenderTarget(GrContext*, const GrBackendRenderTargetDesc&,
                                                  const SkSurfaceProps*) {
     return NULL;
 }
 
 #endif