[ozone/wayland] Switch to xkb-capable keyboard unittests

Rather than mock'ing a xkb-capable layout engine (MockWaylandXkbLayoutEngine),
this CL makes WaylandKeyboardUnittests xkb-capable.

This will allow us to:
- Unify single WaylandXkbKeyboardLayoutEngine instance cross ozone/wayland
  code (chrome and unittests).
- Get rid of MockWaylandXkbKeyboardLayoutEngine altogether (follow up CL).
- Code unittests more straight forward, being able to
  test capslock, and other modifier combinations way easier.

Last, on the spirit of more unittests, CL also adds one more that
exercises modifiers combination.

BUG=578890

Change-Id: Id2145afbfb19a48bab8427d2bd2d0d618a7dec18
Reviewed-on: https://chromium-review.googlesource.com/868610
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#529580}
3 files changed