cros: Support up/down key in webui for keyboard flow.

- Update keyboard_utils to handle up/down keys;
- Update network drop down to not show popup on up/down keys;
- Add up/down key released accelerator for moving focus for Gaia iframe;

BUG=329067

Review URL: https://codereview.chromium.org/101283005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242183 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242183