Allow GMs to be used as benchmarks. Make convex_poly_clip opt in.

R=reed@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/151843002

git-svn-id: http://skia.googlecode.com/svn/trunk@13279 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/gm.h b/gm/gm.h
index e69cfc0..bb20c9c 100644
--- a/gm/gm.h
+++ b/gm/gm.h
@@ -49,6 +49,8 @@
             kSkipPDFRasterization_Flag  = 1 << 8,
 
             kGPUOnly_Flag               = 1 << 9,
+
+            kAsBench_Flag               = 1 << 10, // Run the GM as a benchmark in the bench tool
         };
 
         void draw(SkCanvas*);