Fix WebTransport session cleanup on tab close and explicit disconnect

This CL fixes an issue affecting conferencing applications built
on WebTransport. When a user closes their browser tab during an
active call, the WebTransport connection doesn't properly send a
close frame to the server, causing the server to wait up to 30
seconds before cleaning up the session.

The CL also addresses a related issue where programmatically
closing the WebTransport connection (e.g., by calling an explicit
Close() method in the UI) could sometimes lead to a "double-close"
situation. In this scenario, both the explicit close and
subsequent internal cleanup would attempt to close the connection,
risking crashes or undefined behavior. This fix ensures that
session close is only performed once, regardless of how the
connection is terminated.

Bug: 433421599
Change-Id: I5f73f79bd47f2aa392b4519c83df8ff2dd60d60b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7005507
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Himanshu Panwar <himpanwar@microsoft.com>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529332}