cros: Fix Shortcut Viewer labels for non-Qwerty layout

Currently for non-Qwerty keyboard layout, Shortcut Viewer returns
incorrect keys. This cl fixes the bug by creating a reverse mapping from
dom_code to {dom_key, key_code}.

Bug: 841670
Test: manually.
Change-Id: Ib75bf84b34c0b03bf5501fab0586db8e750478bc
Reviewed-on: https://chromium-review.googlesource.com/1058080
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Kevin Schoedel <kpschoedel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559200}
4 files changed