Spin off GM::runAsBench() from flags.

This will let us kill flags.

BUG=skia:

Review URL: https://codereview.chromium.org/873753002
diff --git a/gm/variedtext.cpp b/gm/variedtext.cpp
index dddd79e..dba5d70 100644
--- a/gm/variedtext.cpp
+++ b/gm/variedtext.cpp
@@ -139,8 +139,9 @@
     uint32_t onGetFlags() const SK_OVERRIDE {
         // The aa hairline stroked rects used to visualize the clip draw slightly differently in
         // quilt mode in dm.
-        return kAsBench_Flag | kSkipTiled_Flag;
+        return kSkipTiled_Flag;
     }
+    bool runAsBench() const SK_OVERRIDE { return true; }
 
 private:
     static const int kCnt = 30;