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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.

Bug: none
Change-Id: I5539b0b9d11a534d059107f57dd91d0f65b7a0a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2420778
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809159}