X11Window: implement event handling and modifiers BUG=angleproject:892 Change-Id: I65f251abdd28b273e2d3d574825f20bba8bb2532 Reviewed-on: https://chromium-review.googlesource.com/270452 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>