Add debug sentinel to GrMemoryPool to check for memory stomping

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 50 matching lines @@
         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
     };
 
+    static const uint32_t kAssignedMarker = 0xCDCDCDCD;
+    static const uint32_t kFreedMarker    = 0xEFEFEFEF;
+
+    struct AllocHeader {
+#ifdef SK_DEBUG
+        uint32_t fSentinal;      ///< known value to check for memory stomping (e.g., (CD)*)
+#endif
+        BlockHeader* fHeader;    ///< pointer back to the block header in which an alloc resides
+    };
+
     enum {
         // We assume this alignment is good enough for everybody.
         kAlignment    = 8,
         kHeaderSize   = GR_CT_ALIGN_UP(sizeof(BlockHeader), kAlignment),
-        kPerAllocPad  = GR_CT_ALIGN_UP(sizeof(BlockHeader*), kAlignment),
+        kPerAllocPad  = GR_CT_ALIGN_UP(sizeof(AllocHeader), kAlignment),
     };
     size_t                            fSize;
     size_t                            fPreallocSize;
     size_t                            fMinAllocSize;
     BlockHeader*                      fHead;
     BlockHeader*                      fTail;
 #ifdef SK_DEBUG
     int                               fAllocationCnt;
     int                               fAllocBlockCnt;
 #endif
 };
 
 #endif