mhtml_generation_browsertest cleanup

Some misc cleanup in anticipation of additional tests.

The only change that impacts existing function is that GenerateMHTML
and GenerateMHTMLForCurrentPage would previously navigate to the generated
MHTML page. This is not what's intended in TestOriginalVsSavedPage, as
it is supposed to make assertions about the original and generated pages.
So I've removed this navigation in GenerateMHTMLForCurrentPage.

Other changes:
* GenerateMHTML returns information
* TestOriginalVsSavedPage takes an options struct, and will be extended
  later. It also returns information, including data computed by a new
  get_page_info.js file, which allows us to assert some things about the
  page have not changed.
* Added forbidden_substrings, and use it preferentially to
  forbidden_substrings_in_saved_page if there are
  assertions that should hold on the original page too.
* accepted basic style suggestions

Bug: 363289333
Change-Id: Ia49ec479bb0aa668ff28f124a8b82f120dc156f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5873675
Reviewed-by: Min Qin <qinmin@chromium.org>
Auto-Submit: Dan H <harringtond@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1357894}