Don't try to use explicit credentials with schemes that don't support it.

net/http/http_auth_handler.h

Index: net/http/http_auth_handler.h
diff --git a/net/http/http_auth_handler.h b/net/http/http_auth_handler.h
index f4e35a43ff09fe5cab21518ea229d2ec4fda1433..8f237e863f48070a75a6556d211b02cadf304dc2 100644
--- a/net/http/http_auth_handler.h
+++ b/net/http/http_auth_handler.h
@@ -135,6 +135,11 @@ class NET_EXPORT_PRIVATE HttpAuthHandler {
   // TODO(cbentzel): Add a pointer to Firefox documentation about risk.
   virtual bool AllowsDefaultCredentials();
 
+  // Returns whether explicit credentials can be used with this handler.  If
+  // true the user may be prompted for credentials if an implicit identity
+  // cannot be determined.
+  virtual bool AllowsExplicitCredentials();
+
  protected:
   enum Property {
     ENCRYPTS_IDENTITY = 1 << 0,