SkDiscardablePixelRef objects are now marked immutable.

src/lazy/SkDiscardablePixelRef.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 "SkDiscardablePixelRef.h"
 #include "SkDiscardableMemory.h"
 
 SkDiscardablePixelRef::SkDiscardablePixelRef(SkImageGenerator* generator,
                                              const SkImageInfo& info,
                                              size_t size,
                                              size_t rowBytes)
     : fGenerator(generator)
     , fInfo(info)
     , fSize(size)
     , fRowBytes(rowBytes)
     , fDiscardableMemory(NULL) {
     SkASSERT(fGenerator != NULL);
     SkASSERT(fSize > 0);
     SkASSERT(fRowBytes > 0);
+    // The SkImageGenerator contract requires fGenerator to always
+    // decode the same image on each call to getPixels().
+    this->setImmutable();
 }
 
 SkDiscardablePixelRef::~SkDiscardablePixelRef() {
     SkDELETE(fGenerator);
 }
 
 void* SkDiscardablePixelRef::onLockPixels(SkColorTable**) {
     if (fDiscardableMemory != NULL) {
         if (fDiscardableMemory->lock()) {
             return fDiscardableMemory->data();
@@ skipping 26 matching lines @@
         SkDELETE(generator);
         return false;
     }
     SkAutoTUnref<SkDiscardablePixelRef> ref(SkNEW_ARGS(SkDiscardablePixelRef,
                                                    (generator, info,
                                                     dst->getSize(),
                                                     dst->rowBytes())));
     dst->setPixelRef(ref);
     return true;
 }