Disable Glic WebUI when policy is disabled at runtime

chrome://glic is already disabled by returning false from
WebUIConfig::IsWebUIEnabled. However, due to crbug.com/389737044, if the
policy is disabled at runtime, a successful navigation to chrome://glic
before being disabled will have registered the Glic WebUIDataSource. In
this state, navigations will successfully fetch resources, even if
IsWebUIEnabled returns false. Mojo and API connections fail in this
case but it results in a JS errored blank page.

This CL more explicitly disables chrome://glic in the care where the
policy is disabled at runtime by registering an empty WebUIDataSource,
replacing the potentially real Glic data source. Navigations now result
in a network failure, which is what happens if chrome://glic is
navigated only when the policy is disabled.

Bug: 382722218
Change-Id: I63d3b783294b82faf4332986a2309ce7acf47f9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6187267
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1412411}