Use DEF_GM everywhere

BUG=skia:1902

Review URL: https://codereview.chromium.org/1518893002
diff --git a/gm/concavepaths.cpp b/gm/concavepaths.cpp
index 37635f8..4008a64 100644
--- a/gm/concavepaths.cpp
+++ b/gm/concavepaths.cpp
@@ -388,5 +388,4 @@
     typedef skiagm::GM INHERITED;
 };
 
-static skiagm::GM* F0(void*) { return new ConcavePathsGM; }
-static skiagm::GMRegistry R0(F0);
+DEF_GM( return new ConcavePathsGM; )