X11 and Ozone: fix use_x11 && use_x11 build for UIControlsAura

When use_x11 && use_ozone are set to true at the same
time, compiler throws an error about multiple definitions
of the CreateUIControlsAura.

Thus, to fix that, create a separate source file for
is_linux case that will choose what class to
use based on the IsUsingOzonePlatform feature flag.

Please note that this is a temp solution and it will
be removed once use_x11 goes away.

PS: this patch uses the same approach used in other patches.
For example, https://crrev.com/c/2247719
PPS: I didn't spot that in the beginning as linux-rel bot
couldn't compile use_x11 && use_ozone because of failures
in the chrome installer step. But after we fixed the sysroot,
I spotted some problems and fixing them now.

Bug: 1085700
Change-Id: I1c2f3a0762df4b195775d30f580df09090f13999
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362698
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Maksim Sisov (GMT+3) <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#799289}

• ui/aura/BUILD.gn[diff]
• ui/aura/test/ui_controls_aurax11.cc[Added - diff]
• ui/aura/test/ui_controls_aurax11.h[Added - diff]
• ui/aura/test/ui_controls_factory_aura_linux.cc[Added - diff]
• ui/aura/test/ui_controls_factory_aurax11.cc[Deleted - diff]
• ui/aura/test/ui_controls_factory_ozone.cc[Deleted - diff]
• ui/aura/test/ui_controls_ozone.cc[Added - diff]
• ui/aura/test/ui_controls_ozone.h[Added - diff]