Move gn _run target generation into test().

ui/events/BUILD.gn

Index: ui/events/BUILD.gn
diff --git a/ui/events/BUILD.gn b/ui/events/BUILD.gn
index a875264491b0c5122c9efc42357552880da63448..c2026f76303990dcb0a10a4b5ffe3d43160157db 100644
--- a/ui/events/BUILD.gn
+++ b/ui/events/BUILD.gn
@@ -333,15 +333,6 @@ source_set("test_support") {
   }
 }
 
-# TODO(GYP): Delete this after we've converted everything to GN.
-# The _run targets exist only for compatibility w/ GYP.
-group("events_unittests_run") {
-  testonly = true
-  deps = [
-    ":events_unittests",
-  ]
-}
-
 test("events_unittests") {
   sources = [
     "android/scroller_unittest.cc",