GitLab

Moves LoadWebAppAndCheckInstallability from being the only non-virtual method
defined in the parent InstallManager class to exist alongside other similar
methods in web_app_install_manager child class.
This also allows it to use the web_app_install_task to run the async parts and
use some shared machinery.
This is a step towards unifying and de-duplicating various web app loading,
checking, and installing steps.

In prep for changing behaviour in http://crrev.com/c/1847609

Bug: 1007860
Change-Id: Ib18356719fc0f321cd6bd100d2ce6ec0a450ee0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873817
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711098}