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Multiple independent subclasses wanted this, so it removes duplication.  It also
moves in the direction of collapsing everything onto one infobar delegate/view,
which I think is the right long-term move.

Most of the CL is reordering the declarations and definitions in subclasses to
match this order.  No intended behavior change.

Bug: none
Change-Id: Iced9e4e8ce17ace87db398a537ead8536b052aab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1983434Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728789}