Gtk3 ui: Add libgtk3ui as a separate build component

chrome/browser/ui/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/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
@@ skipping 1873 matching lines @@
         "views/frame/web_app_left_header_view_ash.cc",
         "views/frame/web_app_left_header_view_ash.h",
         "views/tabs/window_finder_ash.cc",
       ]
     }
   }
   if (use_gio) {
     deps += [ "//build/linux/libgio" ]
   }
   if (use_aura && !use_ozone && is_desktop_linux) {
-    deps += [
-      # gtk2 is the only component that can interact with gtk2 in our new
-      # world.
-      "//chrome/browser/ui/libgtk2ui",
-    ]
+    # These are the only components that can interact with gtk.
+    if (use_gtk3) {
+      deps += [ "//chrome/browser/ui/libgtkui:libgtk3ui" ]
+    } else {
+      deps += [ "//chrome/browser/ui/libgtkui:libgtk2ui" ]
+    }
   }
   if (is_win || is_mac || is_desktop_linux) {
     sources += [
       "avatar_button_error_controller.cc",
       "avatar_button_error_controller.h",
       "avatar_button_error_controller_delegate.h",
       "startup/default_browser_infobar_delegate.cc",
       "startup/default_browser_infobar_delegate.h",
       "startup/default_browser_prompt.cc",
       "startup/default_browser_prompt.h",
@@ skipping 1511 matching lines @@
       "test/test_confirm_bubble_model.cc",
       "test/test_confirm_bubble_model.h",
     ]
     deps += [ "//chrome/test:test_support_ui" ]
   }
 
   if (enable_extensions) {
     deps += [ "//extensions/browser" ]
   }
 }