Make the new Gaia password separated signin flow modal.

chrome/test/base/test_browser_window.h

Index: chrome/test/base/test_browser_window.h
diff --git a/chrome/test/base/test_browser_window.h b/chrome/test/base/test_browser_window.h
index 0e4bfcda3ebba37489a5eb2893a79a2288f3479a..ebe738694d7931d54e800470a1413578a3a8d678 100644
--- a/chrome/test/base/test_browser_window.h
+++ b/chrome/test/base/test_browser_window.h
@@ -148,6 +148,8 @@ class TestBrowserWindow : public BrowserWindow {
   void ShowAvatarBubbleFromAvatarButton(
       AvatarBubbleMode mode,
       const signin::ManageAccountsParams& manage_accounts_params) override {}
+  void ShowModalSigninWindow(AvatarBubbleMode mode) override {}
+  void CloseModalSigninWindow() override {}
   int GetRenderViewHeightInsetWithDetachedBookmarkBar() override;
   void ExecuteExtensionCommand(const extensions::Extension* extension,
                                const extensions::Command& command) override;