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This CL adds a basic test for the uninstall_and_replace feature of
ExternalWebAppManager and fixes two issues:
 - On Chrome OS the uninstall was getting blocked on a user prompt.
 - On non-Chrome OS the uninstall never happened.

The changes in this CL are:
 - Use AppServiceProxy::UninstallSilently() to avoid user prompt.
 - Move ExtensionAppsChromeOs::Uninstall() to ExtensionAppsBase.
 - Instantiate ExtensionApps publisher for non-Chrome OS app service.
 - Refactor ExternalWebAppManager's ParseConfig() out of ScanDir()
   and expose it for testing.

Bug: 809304, 1058265
Change-Id: I86caeeb072ec18a4c16a181a01f3c795d2735648
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2306139
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790233}