Remove PossiblyNull from WTF

WTF::PossiblyNull class had been used in tryFast*alloc API which was removed at r155495, so this is dead code now.

Review URL: https://chromiumcodereview.appspot.com/23774003

git-svn-id: svn://svn.chromium.org/blink/trunk@157282 bbb929c8-8fbe-4397-9dbb-9b2b20218538