Clean up GTK3 build flags.

chrome/browser/ui/libgtkui/BUILD.gn

 # Copyright 2014 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.
 
 assert(is_linux, "This file should only be referenced on Linux")
 
 import("//build/config/features.gni")
-import("//build/config/ui.gni")
+import("//build/config/linux/gtk/gtk.gni")
 import("//printing/features/features.gni")
 
 # gn orders flags on a target before flags from configs. The default config
 # adds -Wall, and these flags have to be after -Wall -- so they need to come
 # from a config and can't be on the target directly.
 config("libgtkui_warnings") {
   if (is_clang) {
     cflags = [
       # G_DEFINE_TYPE automatically generates a *get_instance_private inline
       # function after glib 2.37. That's unused. Prevent to complain about it.
@@ skipping 88 matching lines @@
   "//ui/gfx",
   "//ui/gfx/geometry",
   "//ui/gfx/x",
   "//ui/native_theme",
   "//ui/resources",
   "//ui/shell_dialogs",
   "//ui/strings",
   "//ui/views",
 ]
 
+# Automatically depends on the GTK version associated with the current build
+# flags.
+group("libgtkui") {
+  if (use_gtk3) {
+    public_deps = [
+      ":libgtk3ui",
+    ]
+  } else {
+    public_deps = [
+      ":libgtk2ui",
+    ]
+  }
+}
+
 component("libgtk2ui") {
   sources = common_sources
   configs += common_configs
   defines = [ "LIBGTKUI_IMPLEMENTATION" ]
 
   # GTK2 pulls pangoft2 as dependency, and pangoft2 depends on harfbuzz.
   # To avoid missing indirectly referenced harfbuzz symbols from pango,
   # some hack is required when bundled harfbuzz is used and component build is
   # disabled.
   # See crbug.com/462689 for details.
   all_dependent_configs = [ "//third_party/harfbuzz-ng:pangoft2_link_hack" ]
 
   deps = common_deps + [
            "//build/config/linux/gtk2",
            "//build/config/linux/gtk2:gtkprint2",
          ]
   public_deps = [
     "//chrome/browser:theme_properties",
   ]
 }
 
+# This is compiled with "all" even when it's not referenced to ensure that
+# GTK3 continues to build. GTK3 is explicitly specified by some distros.
 component("libgtk3ui") {
   sources = common_sources
   configs += common_configs
   defines = [ "LIBGTKUI_IMPLEMENTATION" ]
 
   deps = common_deps + [
            "//build/config/linux/gtk3",
            "//build/config/linux/gtk3:gtkprint3",
          ]
   public_deps = [
     "//chrome/browser:theme_properties",
   ]
 }