Make SkGLContext lifetime more well-defined

bench/nanobench.cpp

Index: bench/nanobench.cpp
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index b7858983f8ff6cf95bcc367059e6e5d07eb3d757..827ae037b8a929bac8c5d05ada549620f473fd39 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -175,7 +175,7 @@ struct GPUTarget : public Target {
                                                          SkSurface::kNo_Budgeted, info,
                                                          this->config.samples, &props));
         this->gl = gGrFactory->getContextInfo(this->config.ctxType,
-                                              this->config.ctxOptions)->fGLContext;
+                                              this->config.ctxOptions).fGLContext;
         if (!this->surface.get()) {
             return false;
         }