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This is cleanups to make access control of ToolbarButton's subclasses match
the superclass. in order to do this, reorder some functions and modify comments.
There's no intended behavior change.

Bug: 1048901
Change-Id: I2f956cb40042ab9385d1b7e9203988d7c9a377e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275165Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784445}