Spin off GM::runAsBench() from flags.

This will let us kill flags.

BUG=skia:

Review URL: https://codereview.chromium.org/873753002
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index 6fe2d62..8d00ca1 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -503,7 +503,7 @@
         while (fGMs) {
             SkAutoTDelete<skiagm::GM> gm(fGMs->factory()(NULL));
             fGMs = fGMs->next();
-            if (gm->getFlags() & skiagm::GM::kAsBench_Flag) {
+            if (gm->runAsBench()) {
                 fSourceType = "gm";
                 fBenchType  = "micro";
                 return SkNEW_ARGS(GMBench, (gm.detach()));