Add sentinel to GrMemoryPool's block header

src/gpu/GrMemoryPool.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrMemoryPool_DEFINED
 #define GrMemoryPool_DEFINED
 
@@ skipping 40 matching lines @@
 private:
     struct BlockHeader;
 
     static BlockHeader* CreateBlock(size_t size);
 
     static void DeleteBlock(BlockHeader* block);
 
     void validate();
 
     struct BlockHeader {
+#ifdef SK_DEBUG
+        uint32_t     fBlockSentinal;  ///< known value to check for bad back pointers to blocks
+#endif
         BlockHeader* fNext;      ///< doubly-linked list of blocks.
         BlockHeader* fPrev;
         int          fLiveCount; ///< number of outstanding allocations in the
                                  ///< block.
         intptr_t     fCurrPtr;   ///< ptr to the start of blocks free space.
         intptr_t     fPrevPtr;   ///< ptr to the last allocation made
         size_t       fFreeSize;  ///< amount of free space left in the block.
         size_t       fSize;      ///< total allocated size of the block
     };
 
@@ skipping 18 matching lines @@
     size_t                            fMinAllocSize;
     BlockHeader*                      fHead;
     BlockHeader*                      fTail;
 #ifdef SK_DEBUG
     int                               fAllocationCnt;
     int                               fAllocBlockCnt;
 #endif
 };
 
 #endif