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/convexpolyclip.cpp b/gm/convexpolyclip.cpp
index f7aac89..0ec45b7 100644
--- a/gm/convexpolyclip.cpp
+++ b/gm/convexpolyclip.cpp
@@ -215,9 +215,11 @@
     }
 
     virtual uint32_t onGetFlags() const {
-        return kAsBench_Flag | kSkipTiled_Flag;
+        return kSkipTiled_Flag;
     }
 
+    bool runAsBench() const SK_OVERRIDE { return true; }
+
 private:
     class Clip {
     public: