[Extensions] Inline MessagingDelegate::CreateReceiverForTab()

MessagingDelegate::CreateReceiverForTab() is declared on
MessagingDelegate and then overridden at the //chrome layer by the
ChromeMessagingDelegate. However, this method has no dependencies on
the //chrome layer, and can instead be defined directly on the
MessageService (at the //extensions layer). Do so.

This will make it easier to enable messaging on the experimental
desktop android build.

Bug: 400455005
Change-Id: Id50fc8e15f809dd143f64c566e82285585638705
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6317362
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Auto-Submit: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1427836}