ozone/wayland: Handle preedit string in WaylandInputMethodContext

As of [1], UTF characteres are correctly composed on Ozone/wayland,
taking as input raw key events from InputMethod classes. That is
possible thanks to previously ChromeOS-only CharacterComposer component.
This patch fixes the preedit text handling in WaylandInputMethodContext,
which basically properly support HEX_MODE in CharacterComposer.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1448136

R=rjkroege@chromium.org, shuchen@chromium.org

Bug: 921947
Change-Id: I7cf750df2bf43273464f52a726719d50d74075f8
Reviewed-on: https://chromium-review.googlesource.com/c/1454297
Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Shu Chen <shuchen@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#630679}
3 files changed