"exitPointerLock" is always present on Document, if POINTER_LOCK is compiled on. Unfortunately this means that we can't runtime detect if the feature is compiled but not enabled.
<rdar://problem/29486715>
Created attachment 296202 [details] Patch
Committed r209364: <http://trac.webkit.org/changeset/209364>
*** Bug 165431 has been marked as a duplicate of this bug. ***