Clean up GTK3 build flags.

build/config/linux/gtk/gtk.gni

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Include this file if you need to know at build time whether we're compiling
+# against GTK 2 or 3. But in most cases you can just depend on
+# //build/config/linux/gtk and it will switch for you.
+
+declare_args() {
+  # Whether to compile agains GTKv3 instead of GTKv2.
+  use_gtk3 = false
+}