SkImage_Raster's pixels are always immutable.

src/image/SkImagePriv.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 SkImagePriv_DEFINED
 #define SkImagePriv_DEFINED
 
@@ skipping 21 matching lines @@
  *  It is illegal to call this with a texture-backed bitmap.
  *
  *  If the bitmap's colortype cannot be converted into a corresponding
  *  SkImageInfo, or the bitmap's pixels cannot be accessed, this will return
  *  NULL.
  */
 enum SharedPixelRefMode {
     kLocked_SharedPixelRefMode,
     kUnlocked_SharedPixelRefMode
 };
-extern SkImage* SkNewImageFromRasterBitmap(const SkBitmap&, bool forceSharePixelRef,
-                                           const SkSurfaceProps*, SharedPixelRefMode);
+enum ForceCopyMode {
+    kNo_ForceCopyMode,
+    kYes_ForceCopyMode, // must copy the pixels even if the bitmap is immutable
+};
+extern SkImage* SkNewImageFromRasterBitmap(const SkBitmap&, const SkSurfaceProps*,
+                                           SharedPixelRefMode, ForceCopyMode = kNo_ForceCopyMode);
 
 static inline size_t SkImageMinRowBytes(const SkImageInfo& info) {
     size_t minRB = info.minRowBytes();
     if (kIndex_8_SkColorType != info.colorType()) {
         minRB = SkAlign4(minRB);
     }
     return minRB;
 }
 
 // Given an image created from SkNewImageFromBitmap, return its pixelref. This
 // may be called to see if the surface and the image share the same pixelref,
 // in which case the surface may need to perform a copy-on-write.
 extern const SkPixelRef* SkBitmapImageGetPixelRef(const SkImage* rasterImage);
 
 // When a texture is shared by a surface and an image its budgeted status is that of the
 // surface. This function is used when the surface makes a new texture for itself in order
 // for the orphaned image to determine whether the original texture counts against the
 // budget or not.
 extern void SkTextureImageApplyBudgetedDecision(SkImage* textureImage);
 
 // Update the texture wrapped by an image created with NewTexture. This
 // is called when a surface and image share the same GrTexture and the
 // surface needs to perform a copy-on-write
 extern void SkTextureImageSetTexture(SkImage* image, GrTexture* texture);
 
 GrTexture* GrDeepCopyTexture(GrTexture* src, bool isBudgeted);
 
 #endif