Bug 454916 - [UI Monitoring] wrongly logs UI freeze when view menu is
open on Windows

- PostExternalEventDispatch now starts the freeze timer only if inside
an event.
- Fixed initial synchronization of the event nesting level.
- Improved tracing.

Change-Id: Ieead971f7f42f14114e8590bd01f1e6b994b9495
Signed-off-by: Sergey Prigogin <eclipse.sprigogin@gmail.com>