Add initial UI for glic offline panel.

This enables the outer webui page to close the glic window, and replaces
the guest webview with an offline notice whenever the network status is
offline.

Additional work is ongoing to only show this before the webview is
initialized, and to show a loading UI when the network is online until
the webview is loaded and initialized.

Bug: 379165101
Change-Id: I77273b5a69cbaf65fbe066a2792623b23888c308
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6182938
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Ian Wells <iwells@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1410072}