small fix for nanobench segfault when not running any tests

bench/nanobench.cpp

Index: bench/nanobench.cpp
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index 920979cbf9e66c865f2aa21922344ebaccc4e262..5e059328adae5e9fae805da2074a035debac9a2a 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -894,6 +894,12 @@ int nanobench_main() {
     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;
 }