WebTransport: Implement WebTransport reliability attributes

Expose WebTransport.reliability with the state transitions required by
the WebTransport specification. The value starts as pending and changes
to supports-unreliable after Chromium establishes its HTTP/3 transport.

Also expose WebTransport.supportsReliableOnly as false because Chromium
does not currently implement WebTransport over a reliable-only
transport. Gate both attributes behind the experimental
WebTransportReliability runtime feature. Update the IDL harness,
Web-exposed interface, and Android WebView expectations.

chrome status entry - https://chromestatus.com/feature/5094058497277952

Bug: 545636884
Change-Id: I73f603e6d43506efe2a5e1de1e1c77fc90c12bad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8237894
Reviewed-by: Nidhi Jaju <nidhijaju@chromium.org>
Commit-Queue: Monica Chintala <monicach@microsoft.com>
Reviewed-by: Liang Zhao <lzhao@microsoft.com>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1690895}