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This change switches the extensions menu to enable focus traversal at
the focus manager level, allowing both right/left and up/down key
traversal.

It also removes
BubbleDialogDelegateView::EnableUpDownKeyboardAccelerators() - which is
only used in one other place - in favor of
FocusManager::set_arrow_key_traversal_enabled(), which also supports
left/right arrows and pre-exists the bubble dialog method.

These changes mean that when the scroll pane is active in either dialog,
up and down will scroll the pane but left and right arrow keys will
still allow the user to traverse the dialog (at least on Windows and
Linux; on Mac, up/down may still work within the pane).

Bug: 1140632
Change-Id: I8ea5bc79e9b9ed2595e9503be1221843c50a7b3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487904Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820456}