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Since recently, Chrome started to fail while building for the Ozone/Wayland
backend. The libgtkui::GtkEventLoopX11::GetInstance() symbol is now considered
as undefined by the linker, if the target is configured to run on Wayland.

CL fixes the link-time error by moving the X11-related code of the library
(particularly, inside the gtk_ui.cc source file) under the newly introduced
USE_GTK_EVENT_LOOP_X11 conditional compilation flag where checking on whether
a target platform is Ozone/X11 is performed.

Test: configure Chrome browser for running on Wayland and build the target.

Bug: 1011277
Change-Id: I0236f92e05c2525cd72e2643ecd4772c059e27ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1843972
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703854}