Move gtk-specific browser main parts stuff to its own file.

chrome/browser/ui/libgtk2ui/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")
 
 # gn orders flags on a target before flags from configs. The default config
@@ skipping 11 matching lines @@
     ]
   }
 }
 
 component("libgtk2ui") {
   sources = [
     "app_indicator_icon.cc",
     "app_indicator_icon.h",
     "app_indicator_icon_menu.cc",
     "app_indicator_icon_menu.h",
+    "chrome_browser_main_extra_parts_views_gtk.cc",
+    "chrome_browser_main_extra_parts_views_gtk.h",
     "chrome_gtk_frame.cc",
     "chrome_gtk_frame.h",
     "chrome_gtk_menu_subclasses.cc",
     "chrome_gtk_menu_subclasses.h",
     "gtk2_event_loop.cc",
     "gtk2_event_loop.h",
     "gtk2_key_bindings_handler.cc",
     "gtk2_key_bindings_handler.h",
     "gtk2_status_icon.cc",
     "gtk2_status_icon.h",
@@ skipping 82 matching lines @@
       "//build/config/linux/gtk3",
       "//build/config/linux/gtk3:gtkprint3",
     ]
   } else {
     deps += [
       "//build/config/linux/gtk2",
       "//build/config/linux/gtk2:gtkprint2",
     ]
   }
 }