Change profile_creator to not subclass page_test.

Change profile_creator to not subclass page_test.

No intended behavioral change. This CL is purely a refactor.

Previously, common logic for profile generation was held in
profile_generator.py, and subclasses of profile_creator were merely different
page_tests. This structure has two problems:

- It relies on the assumption that the logic for generating profiles is
relatively similar, modulo some differences in page navigation. There are
currently only 2 subclasses of profile_creator, and they are already diverging
in behavior. I will soon be adding more.

- It doesn't allow for composition of profile generation. In the near future,
there will be a profile creator that takes an existing profile, and adds 25
extensions to it. Another profile creator will take that profile, and add 100
bookmarks. Another one will then perform 1000 navigations.

BUG=444230, 442546

Review URL: https://codereview.chromium.org/825703003

Cr-Commit-Position: refs/heads/master@{#310856}