Add sentinel to GrMemoryPool's block header

src/gpu/GrMemoryPool.h

Index: src/gpu/GrMemoryPool.h
diff --git a/src/gpu/GrMemoryPool.h b/src/gpu/GrMemoryPool.h
index 1dd1732ea97edad53115cc835a639b81a1eaf3a7..43826d354a134f8d7bcbf667e395cb2095194fd0 100644
--- a/src/gpu/GrMemoryPool.h
+++ b/src/gpu/GrMemoryPool.h
@@ -58,6 +58,9 @@ private:
     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