desktop-pwa: Merge ReinstallPlaceholderApp into Install

This has a couple of advantages:

1. Reinstalling placeholders becomes part of the queue of tasks
2. We can try to load the URL before uninstalling the app.
3. We can reuse Install and InstallApps semantics i.e. Install() happens
   immediately but InstallApps() gets queued.

An alternative approach would be to have ReinstallPlaceholderApp() that
clients can use. But we would need a significant refactor of
PendingBookmarkAppManager to do this.

Bug: 844278
Change-Id: I0284cc7e4a51d028cb2ac5b890a813f3f6991b9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566366
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651149}