Gtk3 ui: Add libgtk3ui as a separate build component

chrome/test/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.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/common/features.gni")
@@ skipping 4071 matching lines @@
       "//chrome/browser/profiles/profile_statistics_unittest.cc",
       "//chrome/browser/ui/startup/default_browser_infobar_delegate_unittest.cc",
       "//chrome/browser/ui/startup/startup_browser_creator_impl_unittest.cc",
       "//chrome/browser/ui/startup/startup_tab_provider_unittest.cc",
     ]
   }
   if (use_gio) {
     deps += [ "//build/linux/libgio" ]
   }
   if (!is_chromeos && !use_ozone && is_linux) {
-    deps += [ "//chrome/browser/ui/libgtk2ui" ]
+    if (use_gtk3) {
+      deps += [ "//chrome/browser/ui/libgtk2ui:libgtk3ui" ]
+    } else {
+      deps += [ "//chrome/browser/ui/libgtk2ui:libgtk2ui" ]
+    }
   }
 
   if (enable_task_manager) {
     sources += [
       "../browser/task_manager/providers/browser_process_task_unittest.cc",
       "../browser/task_manager/providers/child_process_task_unittest.cc",
       "../browser/task_manager/sampling/shared_sampler_unittest_win.cc",
       "../browser/task_manager/sampling/task_manager_impl_unittest.cc",
       "../browser/task_manager/task_manager_observer_unittest.cc",
       "../browser/task_manager/test_task_manager.cc",
@@ skipping 924 matching lines @@
       "touch_hud",
       "ui",
     ]
 
     if (is_linux && !is_android) {
       deps += [ "//components/font_service:manifest" ]
       packaged_services += [ "font_service" ]
     }
   }
 }