webkitPointerLockElement returns null when pointer lock request is pending.

https://bugs.webkit.org/show_bug.cgi?id=91186

Reviewed by Dimitri Glazkov.

Source/WebCore:

Script should wait for a pointerlockchange event before detecting
if it has acquired lock. However, if a script attempted to poll
pointerLockElement it could be confused when lock was still pending.
This change ensures that if lock is not yet acquired then
pointerLockElement will return null.

Test: pointer-lock/pointerlockelement-null-when-pending.html

* dom/Document.cpp:
(WebCore::Document::webkitPointerLockElement):
* page/PointerLockController.cpp:
(WebCore::PointerLockController::requestPointerLock):
(WebCore::PointerLockController::elementRemoved):
(WebCore::PointerLockController::documentDetached):
(WebCore::PointerLockController::lockPending):
(WebCore):
(WebCore::PointerLockController::didAcquirePointerLock):
(WebCore::PointerLockController::didNotAcquirePointerLock):
(WebCore::PointerLockController::didLosePointerLock):
(WebCore::PointerLockController::clearElement):
* page/PointerLockController.h:
(PointerLockController):

LayoutTests:

* pointer-lock/locked-element-iframe-removed-from-dom-expected.txt:
* pointer-lock/locked-element-iframe-removed-from-dom.html:
     Updated to reflect new behavior of null returned when lock is pending.
* pointer-lock/pointerlockelement-null-when-pending-expected.txt: Added.
* pointer-lock/pointerlockelement-null-when-pending.html: Added.
     New test specifically for testing null return when lock is pending.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@127606 268f45cc-cd09-0410-ab3c-d52691b4dbfc