aura: Have aura and aura_demo build and run without gtk on linux.

ui/ui.gyp

Index: ui/ui.gyp
diff --git a/ui/ui.gyp b/ui/ui.gyp
index ce8f495f277cb989985b2734ea441ca8b8761123..f031b762a69fa20d405c0471b22cb30fb265cc57 100644
--- a/ui/ui.gyp
+++ b/ui/ui.gyp
@@ -305,6 +305,8 @@
             ['exclude', 'gfx/screen_win.cc'],
             ['exclude', 'base/win/mouse_wheel_util.cc'],
             ['exclude', 'base/win/mouse_wheel_util.h'],
+            ['exclude', 'base/x/active_window_watcher_x.cc'],
+            ['exclude', 'base/x/active_window_watcher_x.h'],
            ],
         }],
         ['use_glib == 1', {
@@ -359,6 +361,9 @@
             'gfx/gtk_util.cc',
             'gfx/gtk_util.h',
           ],
+          'sources!': [
+            'base/clipboard/clipboard_aura.cc',
+          ],
         }],
         ['use_wayland == 1', {
           'sources/': [