harden requirements on SkBitmapCache

src/lazy/SkCachingPixelRef.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCachingPixelRef.h"
 #include "SkBitmapCache.h"
 
@@ skipping 36 matching lines @@
     if (!SkBitmapCache::Find(this->getGenerationID(), info.fWidth, info.fHeight, &fLockedBitmap)) {
         // Cache has been purged, must re-decode.
         if (!fLockedBitmap.allocPixels(info, fRowBytes)) {
             fErrorInDecoding = true;
             return false;
         }
         if (!fImageGenerator->getPixels(info, fLockedBitmap.getPixels(), fRowBytes)) {
             fErrorInDecoding = true;
             return false;
         }
+        fLockedBitmap.setImmutable();
         SkBitmapCache::Add(this->getGenerationID(), info.fWidth, info.fHeight, fLockedBitmap);
     }
 
     // Now bitmap should contain a concrete PixelRef of the decoded image.
     void* pixels = fLockedBitmap.getPixels();
     SkASSERT(pixels != NULL);
     rec->fPixels = pixels;
     rec->fColorTable = NULL;
     rec->fRowBytes = fLockedBitmap.rowBytes();
     return true;
 }
 
 void SkCachingPixelRef::onUnlockPixels() {
     fLockedBitmap.reset();
 }