DevTools: add missing shortcuts on Shortcuts screen

The patch adds missing Alt-Up/Alt-Down shortcuts which change css value
in styles sidebar pane by 0.1.

BUG=397996

Review URL: https://codereview.chromium.org/432803002

git-svn-id: svn://svn.chromium.org/blink/trunk@179324 bbb929c8-8fbe-4397-9dbb-9b2b20218538