[Extensions Cleanup] Move creation of tab-based ports to factory method

ExtensionMessagePorts can be created for tabs, extensions, or a specific
endpoint. The latter two have separate factory methods, but the former
uses a special constructor. This is confusing, since it's inconsistent
and that constructor should only be used to construct tab-based message
ports.

Clean this up by making that a factory method.

This CL has no behavior change.

Bug: None
Change-Id: Ief8d356bf8d396f37c6bc3131ff7b2676a54c9d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6143725
Auto-Submit: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1403274}