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[breadcrumb] Use <cr-action-menu> for the breadcrumb drop-down menu

Use <cr-action-menu> for the drop-down menu items, style to match non-
files-ng menus items (or as close as I could get them).

Tune the <cr-action-menu> location to be under elider ... button, then
tune its horizontal Location for RTL/LTR. <cr-action-menu> is position
absolute and its code needs to calculate widths causing style resolves
so our use of offsetLeft offsetWidth etc to locate the menu _does not_
add more style resolves overall.

Change the breadcrumb HTML structure (simplify and remove unneeded DOM
elements, p:before, p), and JS code to integrate <cr-action-menu>, and
have it deal with drop-down menu keyboard/mouse/scroll interactions.

Note: <cr-action-menu> handles menu item ARIA. Logical follow-up would
be ARIA for the elider ... button itself and consider if we should use
<cr-button>, deal with the elider button tap-target size ...

Bug: 1035691
Change-Id: Ie241cec3911847be90971d13629fbdf47c54e1d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094057Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748085}