[iOS] Add JavaScriptFeature to compute heuristic for DOM distillation eligibility.

Uses the same heuristic as Chrome Android to determine whether DOM
distillation is eligible for the current page. This will be used to
collect metrics on page distillation eligibility.

See crrev.com/c/2911296 for details on the last implementation of this
heuristic.

Bug: 399378832
Change-Id: I7506556fd1ec8e15390567a4b6d3dfa0e84111e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6305247
Commit-Queue: Nohemi Fernandez <fernandex@google.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432617}