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The Plugin VM installation flow currently is driven by the UI side, with
it calling into the installer after each step to start the next step.
This CL changes this so after it starts the installation, the UI side
doesn't need to call into the installer (aside from cancelling) and just
needs to update what it displays to the user.

This makes the flow easier to follow and reduces the coupling between
the UI and the installer backend. It also makes it easier to make
changes to the flow, for example adding additional steps.

Bug: 1038816
Change-Id: Ice6a32a572c16a2bc32e86c166ff6e486791d877
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989722Reviewed-by: Julian Watson <juwa@google.com>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730058}