Release surface between runs to avoid seg fault

bench/GrMipMapBench.cpp

Index: bench/GrMipMapBench.cpp
diff --git a/bench/GrMipMapBench.cpp b/bench/GrMipMapBench.cpp
index 3b4db2459612a2dd4f783067738ec628e4d81bcf..b8fc1ba467a9b8af4cb7212c067c4baec52982d8 100644
--- a/bench/GrMipMapBench.cpp
+++ b/bench/GrMipMapBench.cpp
@@ -59,6 +59,10 @@ protected:
         }
     }
 
+    void onPerCanvasPostDraw(SkCanvas*) override {
+        fSurface.reset(nullptr);
+    }
+
 private:
     typedef Benchmark INHERITED;
 };