Decouple X11WindowEventFilter into WindowEventFilter parent class.
This CL decouples X11WindowEventFilter into two classes: WindowEventFilter
and X11WindowEventFilter. X11WindowEventFilter would inherit from WindowEventFilter
and continued instantiated/used by stock Chrome/Chromium Linux/X11 builds,
as it is today, so there would be no functionality change here.
However, on the other side, downstream forks of Chromium could benefit from
the window system agnostic parts of the implementation, in the newly added
WindowEventFilter class. For instance, the Chromium Ozone/Mus implementation in
 would greatly benefit from it in other to provide resize and window dragging
FWIW, Ozone backends like X11 and Wayland already work with it downstream.