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Remove DISALLOW_COPY, use constexpr more, move constants as close to
first use as possible, etc.

This was originally motivated by wanting to change the
Combobox::set_callback() call to set_closure(); that's present here, but
I couldn't help cleaning up at least some of the rest of this too.

Switching to layout managers (from explicit positioning) also fixes some
graphical bugs that resulted from comboboxes' preferred sizes changing
after creation.

Bug: none
Change-Id: Iad0d483327a07ad865b252f5ab58e90037512125
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421029
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809163}