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 75de1717e22f5b811729f4ae0c7d3a8ee785cf6e..f5f811c633c83c5ebe1a1808a604288de76aaf13 100644
--- a/chrome/test/base/test_browser_window.h
+++ b/chrome/test/base/test_browser_window.h
@@ -146,6 +146,7 @@ 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;