Add a group policy controlling which sites can install extensions.

chrome/browser/extensions/extension_prefs.h

Index: chrome/browser/extensions/extension_prefs.h
diff --git a/chrome/browser/extensions/extension_prefs.h b/chrome/browser/extensions/extension_prefs.h
index 92ca408b3870c73b9e4093636080e3419eadf6ba..37607bfe5e01f14efdaa14162ba5a37a5048c980 100644
--- a/chrome/browser/extensions/extension_prefs.h
+++ b/chrome/browser/extensions/extension_prefs.h
@@ -19,6 +19,7 @@
 #include "chrome/browser/extensions/extension_scoped_prefs.h"
 #include "chrome/browser/extensions/management_policy.h"
 #include "chrome/common/extensions/extension.h"
+#include "chrome/common/extensions/url_pattern_set.h"
 #include "chrome/common/string_ordinal.h"
 
 class ExtensionPrefValueMap;
@@ -432,6 +433,10 @@ class ExtensionPrefs : public extensions::ContentSettingsStore::Observer,
     return extension_sorting_.get();
   }
 
+  // Describes the URLs that are able to install extensions. See
+  // prefs::kExtensionAllowedInstallSites for more information.
+  URLPatternSet GetAllowedInstallSites();
+
  protected:
   // For unit testing. Enables injecting an artificial clock that is used
   // to query the current time, when an extension is installed.