Switches internals of BasePage and some getters to references that are
guaranteed non-null.

Bug: v8:11822
Change-Id: I484c4451720dc7e04f8b89dbe4fef03a3eaf817e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2917038Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74756}