[XProto] Fix X11KeyboardImpl::ChangeKeyMapping

This fixes a regression after [1].  The issue is that
GetKeySymsForUnicode() doesn't handle all characters.  This CL ports
the relevant part of XStringToKeysym() to match the behavior before the
regression.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2436787/5/remoting/host/linux/x11_keyboard_impl.cc#108

R=rkjnsn

Bug: 1144251
Change-Id: I91a3d42d0029e57c9e5a8cb8ee74b7bc76c00100
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518282
Reviewed-by: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823782}