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/circularclips.cpp b/gm/circularclips.cpp
index 6a8415c..69e8b69 100644
--- a/gm/circularclips.cpp
+++ b/gm/circularclips.cpp
@@ -26,9 +26,11 @@
 
 protected:
     uint32_t onGetFlags() const SK_OVERRIDE {
-        return kSkipTiled_Flag | kAsBench_Flag;
+        return kSkipTiled_Flag;
     }
 
+    bool runAsBench() const SK_OVERRIDE { return true; }
+
     SkString onShortName() SK_OVERRIDE {
         return SkString("circular-clips");
     }