Handle the case when the GrResourceCache timestamp wraps.

src/gpu/GrResourceCache.h

Index: src/gpu/GrResourceCache.h
diff --git a/src/gpu/GrResourceCache.h b/src/gpu/GrResourceCache.h
index 3e5df38d7729fabd3edb920be4ef22555366ea13..8331bf5d1642b44e82b813d5ab2a800719076732 100644
--- a/src/gpu/GrResourceCache.h
+++ b/src/gpu/GrResourceCache.h
@@ -171,6 +171,9 @@ public:
     void dumpStats(SkString*) const;
 #endif
 
+    // This function is for unit testing and is only defined in test tools.
+    void changeTimestamp(uint32_t newTimestamp);
+
 private:
     ///////////////////////////////////////////////////////////////////////////
     /// @name Methods accessible via ResourceAccess
@@ -192,6 +195,8 @@ private:
     void removeFromNonpurgeableArray(GrGpuResource*);
     bool overBudget() const { return fBudgetedBytes > fMaxBytes || fBudgetedCount > fMaxCount; }
 
+    uint32_t getNextTimestamp();
+
 #ifdef SK_DEBUG
     bool isInCache(const GrGpuResource* r) const;
     void validate() const;