Change implementation of flush-count based GrGpuResource purging

include/gpu/GrGpuResource.h

Index: include/gpu/GrGpuResource.h
diff --git a/include/gpu/GrGpuResource.h b/include/gpu/GrGpuResource.h
index 9305c167587d71118a3f62ef1fd0bbfc04c92b0f..46e48fc4ee10f41bb255cf155e0707811c6427ae 100644
--- a/include/gpu/GrGpuResource.h
+++ b/include/gpu/GrGpuResource.h
@@ -287,6 +287,7 @@ private:
     // This value reflects how recently this resource was accessed in the cache. This is maintained
     // by the cache.
     uint32_t                    fTimestamp;
+    uint32_t                    fExternalFlushCntWhenBecamePurgeable;
 
     static const size_t kInvalidGpuMemorySize = ~static_cast<size_t>(0);
     GrScratchKey                fScratchKey;