According to ortuno: this won't be a problem *until* we set the source of policy apps in  Issue 876173 .

But I think we'll need to do that, so we also need to fix this.

https://chromium-review.googlesource.com/c/chromium/src/+/1179115 "Keep a web_apps.extension_ids prefs-backed roster" should fix this.

Moving this to P2, since this isn't actually blocking Default Installed Apps.

To clarify, the extension system only tries to uninstall "External" extensions. Policy installed apps are considered "External" but because of the way we install default apps, default apps are not considered "External". So default apps don't have to worry about being uninstalled by the extensions system.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2185228682af3207241406f1867ea6112dde9010

commit 2185228682af3207241406f1867ea6112dde9010
Author: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Date: Tue Aug 28 12:48:04 2018

Keep a web_apps.extension_ids prefs-backed roster

Future commits will need it, as installing web apps from their URL will
need to know whether or not those web apps (and their underlying
extensions) are already installed.

Based on nigeltao's https://crrev.com/c/1179115

But rebased on top of latest CLs and added testing.

TBR=rdevlin.cronin@chromium.org

Bug:  876577 
Change-Id: Ic3e837efe8fb2f79e30a2ffb044f6e02e988d20b
Reviewed-on: https://chromium-review.googlesource.com/1192462
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586669}
[modify] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/browser/extensions/extension_service.cc
[modify] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/browser/web_applications/extensions/BUILD.gn
[modify] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/browser/web_applications/extensions/pending_bookmark_app_manager.cc
[modify] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/browser/web_applications/extensions/pending_bookmark_app_manager.h
[modify] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/browser/web_applications/extensions/pending_bookmark_app_manager_unittest.cc
[add] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/browser/web_applications/extensions/web_app_extension_ids_map.cc
[add] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/browser/web_applications/extensions/web_app_extension_ids_map.h
[modify] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/browser/web_applications/web_app_provider.cc
[modify] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/common/pref_names.cc
[modify] https://crrev.com/2185228682af3207241406f1867ea6112dde9010/chrome/common/pref_names.h