Create Document exitPointerLock and pointerLockElement API methods.

Part of the larger task of creating an unprefixed Pointer Lock API.
Intent to Implement and Ship approved April 21 2014
http://goo.gl/uQTJjH.

BUG=359738

Review URL: https://codereview.chromium.org/279313003

git-svn-id: svn://svn.chromium.org/blink/trunk@174201 bbb929c8-8fbe-4397-9dbb-9b2b20218538