[Extensions WebIDL] Emit Error if top of file hit when parsing comment

When processing comments from a WebIDL file we scan backwards through
lines above an IDL node looking for '//' style comments. This CL adds an
error to be emitted if during this backwards scan we reach the top of the file, because this should never happen in practice.

Also adds test for this.

Bug: 340297705
Change-Id: Ib246ba6e47ffe0b67baed23d60101d430f843c51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6072500
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Commit-Queue: Tim <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1393051}