Make AppBannerManagerDesktop listen to web app install events

This CL makes AppBannerManagerDesktop listen for web app installs
to determine whether to fire the appinstalled event to the renderer.
Previously it depended on every install UI flow to call OnInstall
because we didn't have this event to listen to internally.

Bug: 956810
Change-Id: If78c86c8c407a8777f59236685c388276ef0f724
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1657083
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669970}