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This CL adds initial IME support to ozone/x11 by reusing libgtkui's
LinuxInputMethodContext{Factory} implementation, which uses gtk-immodule as
backend.

Some logic has been added to be able to support the following scenarios,
possible, from now on, in ozone linux platforms:

(1) LinuxInputMethodContextFactory is implemented inside ozone backend
implementation, it is instantiated set in OzonePlatform::InitializeUI. E.g:
ozone/wayland.

(2) If the ozone platform initialization have not created/set a
LinuxInputMethodContextFactory instance and GtkUi is in place, then it is set
as the global instance. E.g: ozone/x11.

Even though the logic to support the aforementioned use cases is quite tricky,
spread over different layers (e.g: views::LinuxUI, libgtkui::GtkUi,
ui::OzonePlatform), using ifdefs, etc; it should be cleaned up and unified once
X11 migration to Ozone is completed.

It is worth noting that proper IME support for now requires GLib message pump
(at least IBus, for example) being used in UI thread (i.e: use_glib=true).

Bug: 987939, 789065
Change-Id: I700d39eec02c6bbeaafca29c9405e8e0e1ff6ade
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1798263
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698627}