make allocPixels throw on failure

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"
 #include "SkImageGenerator.h"
@@ skipping 92 matching lines @@
     if ((NULL == autoGenerator.get())
         || (!autoGenerator->getInfo(&info))
         || (!dst->setInfo(info))) {
         return false;
     }
     // Since dst->setInfo() may have changed/fixed-up info, we copy it back from that bitmap
     info = dst->info();
 
     SkASSERT(info.colorType() != kUnknown_SkColorType);
     if (dst->empty()) {  // Use a normal pixelref.
-        return dst->allocPixels();
+        return dst->tryAllocPixels();
     }
     SkAutoTUnref<SkDiscardablePixelRef> ref(
         SkNEW_ARGS(SkDiscardablePixelRef,
                    (info, autoGenerator.detach(), dst->rowBytes(), factory)));
     dst->setPixelRef(ref);
     return true;
 }
 
 // This is the public API
 bool SkInstallDiscardablePixelRef(SkImageGenerator* generator, SkBitmap* dst) {
     return SkInstallDiscardablePixelRef(generator, dst, NULL);
 }