[iOS] Add flag for persisting tab APC and inner text experiment

This change adds the feature flag that, when active, enables the
persisting of web state APCs and inner text. Currently, this will be
used to run an experiment to measure the impact on performance and
storage that saving user tab contexts involves.

Bug: 445912549
Change-Id: Iefcd18efe505887618337774dd6704773572be3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6966416
Reviewed-by: Nicolas MacBeth <nicolasmacbeth@google.com>
Reviewed-by: Adam Arcaro <adamta@google.com>
Commit-Queue: Charlotte Cloutier <cloutierc@google.com>
Cr-Commit-Position: refs/heads/main@{#1518677}