Track Windows registry-sideloaded extensions.

This adds a creation flag for extensions which tracks if they
were installed from a potentially untrusted source.

The flag is currently set for extensions which are installed
via the Windows registry from a CRX on the local filesystem.

No enforcement action is being taken yet and no additional metrics are being collected.

BUG=325469

Review URL: https://codereview.chromium.org/479513004

Cr-Commit-Position: refs/heads/master@{#290122}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290122 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/browser/extensions/external_provider_impl.h b/chrome/browser/extensions/external_provider_impl.h
index 0a148a4..a0e960a 100644
--- a/chrome/browser/extensions/external_provider_impl.h
+++ b/chrome/browser/extensions/external_provider_impl.h
@@ -73,6 +73,7 @@
   static const char kKeepIfPresent[];
   static const char kSupportedLocales[];
   static const char kWasInstalledByOem[];
+  static const char kMayBeUntrusted[];
 
   void set_auto_acknowledge(bool auto_acknowledge) {
     auto_acknowledge_ = auto_acknowledge;