[Page Zoom] Prep for fast-follow work to AccessibilityPageZoom

This CL updates the features that are exposed with the experiment:
  AccessibilityPageZoomEnhancements

We will be using this feature flag for the fast-follow work related to
the recently enabled AccessibilityPageZoom experiment. We have created
a new feature flag (AccessibilityPageZoomV2) for some features that will
be split out from the fast-follow work, and will be part of future
experimentation around the OS-level adjustments inside Chrome.

With this CL we move an accessibility settings toggle to the V2 flag,
update related tests, and update the behavior of the 3-dot menu to show
the zoom option for users that have a non-default OS-level for setting
(although we are still not modifying the zoom of the web contents in
those cases). We had initially planned on this approach for the
AccessibilityPageZoom feature, but punted it to fast-follow to minimize
the impact of the initial feature. The fast-follow feature also includes
a Reset button on the main zoom slider, and the Site Settings flow, which
have both been in the code base and guarded by the correct flag.

After UI / content approval we will update any related strings and switch
them to be translateable.

AX-Relnotes: N/A
Bug: 369146064
Change-Id: I473c0d608fc77539f7a386b42d49de6a4c989099
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5882612
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Amanda Lin Dietz <aldietz@google.com>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Wenyu Fu <wenyufu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1359596}