[X11] Fix x11::Connection::HasNextEvent() for XCB-queued events

HasNextEvent() was only checking if an event is present in our own event
queue. With this CL, it also checks XCB's event queue and moves the
event into our own queue if one is available. This fixes certain race
conditions or deadlocks that may arise when the event dispatcher
(X11EventWatcherFdWatch) sees the X11 socket is not readable and there's
no event queued. Note that this issue only occurs on ChromeOS, not
Linux, since Linux uses X11EventWatcherGlib which calls ReadResponses()
before polling.

R=thestig
CC=vincentchiang

Fixed: 405272123
Change-Id: Ie4aed8cd8930a9a97ffe0e9af5991ea4de80777c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6618668
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1470186}