Oilpan: move Page's PointerLockController object to the heap.

Move this Page-owned controller object to the heap, allowing a pair
of Persistent<> references to be removed in the process.

R=ager@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/328133002

git-svn-id: svn://svn.chromium.org/blink/trunk@175957 bbb929c8-8fbe-4397-9dbb-9b2b20218538