Deprecate the External Extension Install via Preferences file (Windows only).

chrome/browser/extensions/external_provider_impl.cc

Index: chrome/browser/extensions/external_provider_impl.cc
diff --git a/chrome/browser/extensions/external_provider_impl.cc b/chrome/browser/extensions/external_provider_impl.cc
index 1a85f5ff1851dfd46f796f3a9f1579457f70c2a1..cb8fbb4a54d93d0c9614eda24dcebf94e57d5fc1 100644
--- a/chrome/browser/extensions/external_provider_impl.cc
+++ b/chrome/browser/extensions/external_provider_impl.cc
@@ -461,6 +461,7 @@ void ExternalProviderImpl::CreateExternalProviders(
 #endif
 
   if (!profile->IsManaged() && !is_chromeos_demo_session) {
+#if !defined(OS_WIN)
     provider_list->push_back(
         linked_ptr<ExternalProviderInterface>(
             new ExternalProviderImpl(
@@ -471,6 +472,7 @@ void ExternalProviderImpl::CreateExternalProviders(
                 Manifest::EXTERNAL_PREF,
                 Manifest::EXTERNAL_PREF_DOWNLOAD,
                 bundled_extension_creation_flags)));
+#endif
 
 #if defined(OS_CHROMEOS) || defined (OS_MACOSX)
     // Define a per-user source of external extensions.