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This is a reland of a798c106

Original change's description:
> Get rid of native event (i.e: XEvent) usage in libgtkui
> 
> Get rid of native event (i.e: XEvent) usage when translating ui::KeyEvent into
> GdkEvent in GtkUi GdkKeyBindingsHandler and InputMethod context implementation.
> Native event usage outside platform code is undesired and blocks changes such
> as Aura/X11 => Ozone/X11 migration (e.g: PlatformEvent migration
> crbug.com/965991).
> 
> It is worth noting that aura IME code has been slightly modified to set root
> window as Event::target() during the PRE_DISPATCH phase (for some reason
> InputMethodContext::DispatchKeyEvent() is called at this phase) so that IME
> context implementation is then able to retrieve the X11 Window id, needed by
> GdkEvent system.
> 
> To be able to retrieve XEvent group field in gtkui code, an Event::Property has
> been added and for now is set only for non-ozone X11. Ozone/X11 will be
> addressed in a follow-up CL.
> 
> Rationale:
> https://docs.google.com/document/d/1Neh2p6f8pew9SPuEAtnJMafozAY7uhAIfhwd43CLETc/edit?usp=sharing
> 
> Bug: 965991, 987939
> Change-Id: I64929d6866c1dfcfddbfe75166a1bad139c2da8d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789603
> 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@{#695827}

Bug: 965991, 987939
Change-Id: I921805b73cd9c55d1e069026091f0526d242b9cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1805156Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#697433}