Replace the WAS_INSTALLED_BY_CUSTODIAN creation flag with a pref

extensions/browser/extension_prefs.h

Index: extensions/browser/extension_prefs.h
diff --git a/extensions/browser/extension_prefs.h b/extensions/browser/extension_prefs.h
index bae2126e755715670b87d5f5e1af6f103330df54..f0671c9639e437f598635d5c8d6203ce143de10c 100644
--- a/extensions/browser/extension_prefs.h
+++ b/extensions/browser/extension_prefs.h
@@ -409,6 +409,13 @@ class ExtensionPrefs : public ExtensionScopedPrefs, public KeyedService {
   void SetAllowFileAccess(const std::string& extension_id, bool allow);
   bool HasAllowFileAccessSetting(const std::string& extension_id) const;
 
+  // Returns true if this extension has been installed by the custodian of
+  // a supervised user. It is relevant for supervised users and used to block
+  // them from uninstalling the extension for example.
+  bool WasInstalledByCustodian(const std::string& extension_id) const;
+  void SetWasInstalledByCustodian(const std::string& extension_id,

[Devlin, 2016/06/13 14:36:58]

We should remove these and just use the generic UpdateExtensionPref() method
from extension_util.

[mamir, 2016/06/13 15:25:31]

But other prefs such as IsIcognitoEnabled, AllowFileAccess ...etc. has their own
Setters.
Why this should be done differently?

[Devlin, 2016/06/13 15:41:43]

We should do everything through Set/Get extension pref, but sadly we don't (we
haven't gone through and converted everything, etc).  If you look at the number
of callers of UpdateExtensionPref, you can see that it's more common (I'm pretty
sure).  And it makes it a lot clearer, especially when you have to do some
surrounding work (as we do in extension_util), because then we don't need the
annoying comments "You almost certainly want to use this other method over here
with the exact same name" (as we say on lines 398, 406, etc).

[mamir, 2016/06/13 18:02:47]

Done.

+                                  bool installed_by_custodian);
+
   // Saves ExtensionInfo for each installed extension with the path to the
   // version directory and the location. Blacklisted extensions won't be saved
   // and neither will external extensions the user has explicitly uninstalled.