Create base for the "Upload to Cloud" setup dialog

This defines a base setup page which contains the common shadow DOM and
code used for each page in the setup flow. Each page inherits from it
and defines its own content by modifying the content inside their own
element.

This also adds the welcome page to the dialog, but it's only there to
demonstrate how a page would be added, and thus is incomplete.

The upload page is ported from the existing code.

Bug: b/251045239
Change-Id: I10a6e9288c464f58ac89f9c62fd467897aeed20e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3960709
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1064683}