Enable "add to..." for chrome-extension:// pages.

Chrome extensions cannot be pinned to taskbar/shelf/desktop when the
new bookmark apps system is enabled. This CL fixes the bug.

BUG=519033

Review URL: https://codereview.chromium.org/1308813004

Cr-Commit-Position: refs/heads/master@{#345270}
diff --git a/extensions/common/extension.h b/extensions/common/extension.h
index c8d1690..c4934d8 100644
--- a/extensions/common/extension.h
+++ b/extensions/common/extension.h
@@ -204,6 +204,9 @@
   // Valid schemes for web extent URLPatterns.
   static const int kValidWebExtentSchemes;
 
+  // Valid schemes for bookmark app installs.
+  static const int kValidBookmarkAppSchemes;
+
   // Valid schemes for host permission URLPatterns.
   static const int kValidHostPermissionSchemes;