Refactoring: Make some Page data members const.

These pointer members are not updated after the Page construction.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23183007

git-svn-id: svn://svn.chromium.org/blink/trunk@156341 bbb929c8-8fbe-4397-9dbb-9b2b20218538