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Consolidate most ChromeOS files in chrome/browser/ui/BUILD.gn.

Also group various enable_foo and use_bar blocks alphabetically at the
end, after the is_quxos and use_bazui sections.

Also remove all -= operator usage and use += in the right sections
instead. Add a few new sections so certain files no longer need to be
included twice/thrice in separate places.

Change-Id: I2f62dcd98ab09d824080c750b3c98d2f457881b9
Reviewed-on: https://chromium-review.googlesource.com/689454
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505396}