refactor-864370: Minor - Use KeyEvent constants
refactor-864370: Minor - Use KeyEvent constants

Until the code re-org, helpers.js was unable to easily require
'gcli/util/util' so we used the inlined constants rather than
KeyEvent.DOM_VK_* constants. Now we’re no longer under that constraint.

Signed-off-by: Joe Walker <7a17872fbb32c7d760c9a16ee3076daead11efcf@mozilla.com>

mozilla/gcli,mozilla/gcli,mozilla/gcli,joewalker/gcli,joewalker/gcli