Virtual Desks: Modify keyboard shortcuts

See issue 970324 for details. The new keyboard shortcuts are:

Search+Shift+=  : New desk.
Search+Shift+-  : Remove desk.
Search+]        : Activate desk on the right (if any).
Search+[        : Activate desk on the left (if any).
Search+Shift+]  : Move active window (or highlighted window in
                  overview) to desk on the right (if any).
Search+Shift+[  : Move active window (or highlighted window in
                  overview) to desk on the left (if any).

for minor change in c/b/ui/ash:
TBR=sky@chromium.org
BUG=866622, 970324

Change-Id: I4ba22219e35f1a07b6714ae88710ed46b06e44dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696903
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677000}