[iOS] Move main parts to ios/chrome/browser/web

It is the only place that is using it.

Bug: 1354614
Change-Id: I232c8001df04348f9c3fecf565f200a7d86f178a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3891016


Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1045869}