[Views] Make MenuButton track its desired state while pressed

ui/views/widget/desktop_aura/desktop_native_widget_aura.cc

Index: ui/views/widget/desktop_aura/desktop_native_widget_aura.cc
diff --git a/ui/views/widget/desktop_aura/desktop_native_widget_aura.cc b/ui/views/widget/desktop_aura/desktop_native_widget_aura.cc
index 88c60906b7268d02bffd85fdb683ef05c9bca77a..e48540bda590229f9cc3f5eec2e52ecbe6d12236 100644
--- a/ui/views/widget/desktop_aura/desktop_native_widget_aura.cc
+++ b/ui/views/widget/desktop_aura/desktop_native_widget_aura.cc
@@ -870,7 +870,7 @@ void DesktopNativeWidgetAura::SetCursor(gfx::NativeCursor cursor) {
 }
 
 bool DesktopNativeWidgetAura::IsMouseEventsEnabled() const {
-  if (!content_window_)
+  if (!content_window_ || !host_)

[sky, 2015/01/24 00:00:36]

This has me worried, and at the least is worth a comment. Why do we need to
check for no host here when no where else in this class do we do it?

[Devlin, 2015/01/24 00:11:48]

Yeah, I'm not real keen on it either.
Reason:
- OnHostClosed() resets() |host_|, and then sets |content_window_| to null.
- |host_|'s destructor works its way around to calling OnWidgetDestroying.
- OnWidgetDestroying is a path for release MenuButton::PressedLocks.
- Releasing the last pressed lock checks if the mouse is hovered, which checks
if IsMouseEventsEnabled() (leads to crash).

In short, checking if mouse events are enabled during a widget shutdown crashes.
 I was wondering about other possible solutions (mark a widget as destroying in
views::Widget, and don't check it if it is? [Seems like a bandaid, not a
solution.]  Set content_window_ to null before resetting host_? [Tried that one;
it fails because destruction/reset order is hyper-sensitive]).

This seemed like the safest solution, because it's common to check something for
nullness before using it (on line 876), but I'd love your opinion.

     return false;
   aura::client::CursorClient* cursor_client =
       aura::client::GetCursorClient(host_->window());