small fix for nanobench segfault when not running any tests

bench/nanobench.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <ctype.h>
 
 #include "Benchmark.h"
@@ skipping 876 matching lines @@
         if (FLAGS_resetGpuContext || FLAGS_abandonGpuContext) {
             gGrFactory->destroyContexts();
         }
 #endif
     }
 
     log->bench("memory_usage", 0,0);
     log->config("meta");
     log->metric("max_rss_mb", sk_tools::getMaxResidentSetSizeMB());
 
+#if SK_SUPPORT_GPU
+    // Make sure we clean up the global GrContextFactory here, otherwise we might race with the
+    // SkEventTracer destructor
+    gGrFactory.reset(NULL);
+#endif
+
     return 0;
 }
 
 #if !defined SK_BUILD_FOR_IOS
 int main(int argc, char** argv) {
     SkCommandLineFlags::Parse(argc, argv);
     return nanobench_main();
 }
 #endif