Revert of Move gn _run target generation into test().

ui/compositor/BUILD.gn

Index: ui/compositor/BUILD.gn
diff --git a/ui/compositor/BUILD.gn b/ui/compositor/BUILD.gn
index db0b0ce4b6c3e054b04767ad4d5714e1664a5fc9..c2c3c225b22450bafc9edf8ec59dda91ced3b0ba 100644
--- a/ui/compositor/BUILD.gn
+++ b/ui/compositor/BUILD.gn
@@ -160,6 +160,15 @@
   }
 }
 
+# TODO(GYP): Delete this after we've converted everything to GN.
+# The _run targets exist only for compatibility w/ GYP.
+group("compositor_unittests_run") {
+  testonly = true
+  deps = [
+    ":compositor_unittests",
+  ]
+}
+
 test("compositor_unittests") {
   sources = [
     "callback_layer_animation_observer_unittest.cc",