Page::contextMenuController() and pointerLockController() should return references.

They can't be null. They had better provide references so that users
don't need to worry about null pointer deferences.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22821004

git-svn-id: svn://svn.chromium.org/blink/trunk@156090 bbb929c8-8fbe-4397-9dbb-9b2b20218538