SkImage_Raster's pixels are always immutable.

src/image/SkSurface_Base.h

Index: src/image/SkSurface_Base.h
diff --git a/src/image/SkSurface_Base.h b/src/image/SkSurface_Base.h
index a52a2b44d92a7be8eee31e114bc6da9a26b6c991..8f0eef3b60e4c854ae0734bff00fc2d11c903b1a 100644
--- a/src/image/SkSurface_Base.h
+++ b/src/image/SkSurface_Base.h
@@ -68,9 +68,17 @@ public:
      */
     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();