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Allow arrow key navigation in the overflow bubble as well, so that
app reordering also works and integrates smoothly with keyboard
navigation in there too.

This does not yet support reordering of app across the "overflow
border" (e.g. last app of main shelf with first app of overflow).

Also refactor a simple |IsArrowKeyCode| function in a place where it's
easy to reuse. Some places in app_list could also use this but that
part of the code has a slightly unusual namespace pattern usage and
I didn't want to mess with BUILD.gn and deps too much.

See this "in action" at:

    https://bugs.chromium.org/p/chromium/issues/detail?id=888637#c28

Bug: 888637
Change-Id: Icfa22870bd603093cd4889ffdea10d91c01abbc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1535304Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644648}