Add sentinel to GrMemoryPool's block header

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Review URL: https://codereview.chromium.org/1562813002
diff --git a/src/gpu/GrMemoryPool.h b/src/gpu/GrMemoryPool.h
index 1dd1732..43826d3 100644
--- a/src/gpu/GrMemoryPool.h
+++ b/src/gpu/GrMemoryPool.h
@@ -58,6 +58,9 @@
     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