Make gtk2/gtk3 compile time switchable.

build/config/linux/gtk2/BUILD.gn

 # Copyright 2015 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.
 
 import("//build/config/linux/pkg_config.gni")
 
 assert(is_linux, "This file should only be referenced on Linux")
 
-# Depend on //build/config/linux/gtk to use GTK.
+# Depend on //build/config/linux/gtk2 to use GTKv2.
 #
 # GN doesn't check visibility for configs so we give this an obviously internal
 # name to discourage random targets from accidentally depending on this and
 # bypassing the GTK target's visibility.
-pkg_config("gtk_internal_config") {
+pkg_config("gtk2_internal_config") {
   # Gtk requires gmodule, but it does not list it as a dependency in some
   # misconfigured systems.
   packages = [
     "gmodule-2.0",
     "gtk+-2.0",
     "gthread-2.0",
   ]
 }
 
 # Basically no parts of Chrome should depend on GTK. To prevent accidents, the
 # parts that explicitly need GTK are whitelisted on this target.
-group("gtk") {
+group("gtk2") {
   visibility = [
     "//chrome/browser/ui/libgtk2ui",
     "//gpu/gles2_conform_support:gles2_conform_test_windowless",
     "//remoting/host",
     "//remoting/host/it2me:remote_assistance_host",
     "//remoting/host:remoting_me2me_host_static",
   ]
-  public_configs = [ ":gtk_internal_config" ]
+  public_configs = [ ":gtk2_internal_config" ]
 }
 
 # Depend on "gtkprint" to get this.
-pkg_config("gtkprint_internal_config") {
+pkg_config("gtkprint2_internal_config") {
   packages = [ "gtk+-unix-print-2.0" ]
 }
 
-group("gtkprint") {
+group("gtkprint2") {
   visibility = [ "//chrome/browser/ui/libgtk2ui" ]
-  public_configs = [ ":gtkprint_internal_config" ]
+  public_configs = [ ":gtkprint2_internal_config" ]
 }