Use DEF_GM everywhere

BUG=skia:1902

Review URL: https://codereview.chromium.org/1518893002
diff --git a/gm/dropshadowimagefilter.cpp b/gm/dropshadowimagefilter.cpp
index 1b6bca4..81824ab 100644
--- a/gm/dropshadowimagefilter.cpp
+++ b/gm/dropshadowimagefilter.cpp
@@ -169,5 +169,4 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
-static skiagm::GM* MyFactory(void*) { return new DropShadowImageFilterGM; }
-static skiagm::GMRegistry reg(MyFactory);
+DEF_GM( return new DropShadowImageFilterGM; )