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Introduce the shortcut item views for Keyboard Shortcut Viewer (KSV).

Changes:
1. Add item view and item list view.
2. For V1, use US keyboard layout to display the shortcut keys.

Bug: 768932
Test: KeyboardShortcutViewTest and can show the shortcut item list.
Change-Id: Ia314dff02f1bec9f8c5cff195a93a3033c9a32f6
Reviewed-on: https://chromium-review.googlesource.com/893604
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533907}