Add ontoggle event handler, and dispatch toggle events to the details
element if the open attribute is added or changed. According to the
spec, if a new toggle event has been queued, previous toggle events
should be aborted.
MozReview-Commit-ID: EN6Jf5hVHHD
--HG--
extra : rebase_source : 35605e49950bb59a0eb6dca594c3ede465ff587d
To retain backward compatibility, <details> tags should not collapse its
children when dom.details_element.enabled = false.
--HG--
extra : rebase_source : 6b47e64672720ffecd23f670c31de1c7d92bee8c
