[ozone/wayland] Use ui::EventModifiers to handle modifiers

CL switches WaylandKeyboard class from using a raw integer to
handle keyboard modifiers to  using ui::EventModifiers class.

Although there is no behavior changed by this CL, it makes it easier
to handle more complex cases, in follow up CLs.

The CL also changes the KeyboardLayoutManager instantiation order in
OzonePlatformWayland::InitializeUI, so that it happens prior to
WaylandConnection instantiation.
Reason: once WaylandConnection creates WaylandKeyboard it sets
its EventModifiers instance in KeyboardLayoutManager, hence the
later needs to be properly instantiated by then.

BUG=578890

Change-Id: I9d589e72917b83c4319c6046e6b1cad8cca079ad
Reviewed-on: https://chromium-review.googlesource.com/832887
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#525749}
5 files changed