GitLab

- Remove the START_DOWNLOADING state in the UI code.
- Rename START_DOWNLOADING_DLC to STARTING and fix the initial state to
be that. While we logically start installation immediately upon opening
the dialog, we actually wait for AddedToWidget() to be called first, so
retaining this state allows us to avoid confusion of being in an active
state without having started the installation.
- Remove the OnDlcDownloadStarted and OnDownloadStarted callbacks, the
installer UI doesn't need these.
- Add TODO suggesting further improvements regarding cancellation.

Change-Id: I83575155047665139f0c1aa2af6b958b52df0b93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1994846
Commit-Queue: Timothy Loh <timloh@chromium.org>
Reviewed-by: Julian Watson <juwa@google.com>
Cr-Commit-Position: refs/heads/master@{#730075}