Implement the new Sync Confirmation dialog on Linux and Windows.

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 c707b4bca6a963d049d53f73dd34f10429fe257c..f3171235130d7f0bd5ca106865cfed6f57e78420 100644
--- a/chrome/test/base/test_browser_window.h
+++ b/chrome/test/base/test_browser_window.h
@@ -147,6 +147,7 @@ class TestBrowserWindow : public BrowserWindow {
       AvatarBubbleMode mode,
       signin_metrics::AccessPoint access_point) override {}
   void CloseModalSigninWindow() override {}
+  void ShowModalSyncConfirmationWindow() override {}
   int GetRenderViewHeightInsetWithDetachedBookmarkBar() override;
   void ExecuteExtensionCommand(const extensions::Extension* extension,
                                const extensions::Command& command) override;