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This adds a helper, UpdateButtons(), to set all buttons' visibility,
text, and enabled state based on the current mode; this replaces the
scattered pieces throughout other functions.

This also removes the places that set |dropdown_state_| to NORMAL.
Since this is supposed to track the actual button state, it only makes
sense to toggle it in places like ButtonPressed() and ReleaseDropdown().
It also doesn't make sense to change this variable directly instead of
going through SetDropdownState(), since then the state and the visual
appearance could be out of sync.

Bug: none
Change-Id: I444fd3b14ab1ec8a4b0aed37ce2d80539d54f199
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299139
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788861}