Added Doc for GetAllocator in SkResourceCache and SkBitmapCache

src/core/SkBitmapCache.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkBitmapCache_DEFINED
 #define SkBitmapCache_DEFINED
 
 #include "SkScalar.h"
 #include "SkBitmap.h"
 
 class SkMipMap;
 
 class SkBitmapCache {
 public:
     /**
-     *  Use this allocator for bitmaps, so they can use ashmem when available.
+     * Use this allocator for bitmaps, so they can use ashmem when available.
+     * Returns NULL if the ResourceCache has not been initialized with a DiscardableFactory.
      */
     static SkBitmap::Allocator* GetAllocator();
 
     /**
      *  Search based on the src bitmap and inverse scales in X and Y. If found, returns true and
      *  result will be set to the matching bitmap with its pixels already locked.
      */
     static bool Find(const SkBitmap& src, SkScalar invScaleX, SkScalar invScaleY, SkBitmap* result);
-    
+
     /*
      *  result must be marked isImmutable()
      */
     static void Add(const SkBitmap& src, SkScalar invScaleX, SkScalar invScaleY,
                     const SkBitmap& result);
 
     /**
      *  Search based on the bitmap's genID, width, height. If found, returns true and
      *  result will be set to the matching bitmap with its pixels already locked.
      */
     static bool Find(uint32_t genID, int width, int height, SkBitmap* result);
 
     /*
      *  result must be marked isImmutable()
      */
     static void Add(uint32_t genID, int width, int height, const SkBitmap& result);
 };
 
 class SkMipMapCache {
 public:
     static const SkMipMap* FindAndRef(const SkBitmap& src);
     static void Add(const SkBitmap& src, const SkMipMap* result);
 };
 
 #endif