[A11yPerformance] Introduce accessibility performance experiment

This CL introduces a new feature to assess the impact of accessibility
changes on the user and their performance.

The experiment includes four flag types:

1. `AX_MODE_COMPLETE_EXPERIMENT`: Enables the complete accessibility
mode, including all of the properties needed by screen readers and other
assistive technologies. 2. `WEB_CONTENTS_ONLY_EXPERIMENT`: Enables
`kAXModeWebContentsOnly` accessibility mode. 3.
`AX_MODE_COMPLETE_NO_INLINE_TEXT_BOXES_EXPERIMENT`: Enables the complete
accessibility mode, excluding inline text boxes. 4.
`RENDERER_SERIALIZATION_ONLY_EXPERIMENT`: Enables a special
accessibility mode that uses `kAXModeComplete`, but only serializes
changes in the renderer and discards them in the browser, thus skipping
the deserialization phase.

The accessibility modes will be activated on startup if there is no
accessibility software enabled.

The experiment flags are logged on the metrics side, and the user that
has these flags are differentiated from the users that have a11y turned
on by themselves.

The main purpose of this change is to measure performance impact of
accessibility changes on a wider scope, not just the AT-users. This
should only have an effect on non-AT users.

Change-Id: I5a37d54cfbe7cfdb3a8ec0a7d7976d214dedc257
Bug: 395095542
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6463139
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
Cr-Commit-Position: refs/heads/main@{#1460285}