Release surface between runs to avoid seg fault

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2032693002

Review-Url: https://codereview.chromium.org/2032693002

bench/GrMipMapBench.cpp

@@ -59,6 +59,10 @@ protected:
         }
     }
 
+    void onPerCanvasPostDraw(SkCanvas*) override {
+        fSurface.reset(nullptr);
+    }
+
 private:
     typedef Benchmark INHERITED;
 };