Release surface between runs to avoid seg fault

bench/GrMipMapBench.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Benchmark.h"
 #include "SkCanvas.h"
 #include "SkImage.h"
@@ skipping 41 matching lines @@
             fSurface->getCanvas()->drawPoint(0, 0, SK_ColorWHITE);
 
             // Draw reduced version of surface to original canvas, to trigger mip generation
             canvas->save();
             canvas->scale(0.1f, 0.1f);
             canvas->drawImage(fSurface->makeImageSnapshot(SkBudgeted::kNo), 0, 0, &paint);
             canvas->restore();
         }
     }
 
+    void onPerCanvasPostDraw(SkCanvas*) override {
+        fSurface.reset(nullptr);
+    }
+
 private:
     typedef Benchmark INHERITED;
 };
 
 // Build variants that exercise the width and heights being even or odd at each level, as the
 // impl specializes on each of these.
 //
 DEF_BENCH( return new GrMipMapBench(511, 1023); )
 DEF_BENCH( return new GrMipMapBench(512, 511); )
 DEF_BENCH( return new GrMipMapBench(511, 512); )
 DEF_BENCH( return new GrMipMapBench(512, 512); )
 
 #endif