Add WebUI for the floating workspace dialog

This dialog reuses some OOBE styles and is supposed
to be shown on the session startup, while floating
workspace is restoring the session.

Screenshot: screenshot/3ErR6vpqEVYQFPF.png

Bug: b:411121762
Change-Id: Id14bfc546a597e2f3b8530d3c5d692ac6fac06e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6532675
Commit-Queue: Michael Slutskii <slutskii@google.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Andrey Davydov <andreydav@google.com>
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460110}