Fix up and down keys bringing up the autocomplete results in popup windows.

The up and down keys currently bring up the autocomplete results in a
non-editable omnibox.

This patch makes the omnibox ignore up and down key events if the omnibox
is read only.

BUG=306362

Review URL: https://codereview.chromium.org/107883003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242850 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242850