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Currently Gdk backend is auto-selected in Ozone, which leads to crashes
when, for example, ozone platform is set to x11 while the browser is
being executed in a Wayland compositor (e.g: Gnome/Wayland for example).

CL modifies GtkUi ctor to set allowed gdk backends based on the current
ozone plarform.

Bug: 1008924
Change-Id: I12163357e4d3d517c4c98672ed9db3c064c2d244
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829319
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701575}