Move views::Accelerator to ui in order to use it from aura code.

views/widget/native_widget_gtk.cc

Index: views/widget/native_widget_gtk.cc
diff --git a/views/widget/native_widget_gtk.cc b/views/widget/native_widget_gtk.cc
index 88a2f6e83d136beb781dffeab8d1be18e9cc1c1b..240c2ad4e9fd6a3c3915632c943effaf7382f444 100644
--- a/views/widget/native_widget_gtk.cc
+++ b/views/widget/native_widget_gtk.cc
@@ -614,7 +614,7 @@ bool NativeWidgetGtk::HandleKeyboardEvent(const KeyEvent& key) {
              (key.flags() & ~ui::EF_ALT_DOWN) == 0) {
     // Trigger VKEY_MENU when only this key is pressed and released, and both
     // press and release events are not handled by others.
-    Accelerator accelerator(ui::VKEY_MENU, false, false, false);
+    ui::Accelerator accelerator(ui::VKEY_MENU, false, false, false);
     handled = GetWidget()->GetFocusManager()->ProcessAccelerator(accelerator);
   }