Firefox overlay is seen opened even if Import Overlay is Cancelled.

chrome/browser/ui/cocoa/importer/import_lock_dialog_cocoa.mm

Index: chrome/browser/ui/cocoa/importer/import_lock_dialog_cocoa.mm
diff --git a/chrome/browser/ui/cocoa/importer/import_lock_dialog_cocoa.mm b/chrome/browser/ui/cocoa/importer/import_lock_dialog_cocoa.mm
index 4aa0fae35581061d1a902563e2b4c322ae58bce0..6b667d6de0002f9d98704d79898a9bdfa7b63d7f 100644
--- a/chrome/browser/ui/cocoa/importer/import_lock_dialog_cocoa.mm
+++ b/chrome/browser/ui/cocoa/importer/import_lock_dialog_cocoa.mm
@@ -36,4 +36,10 @@ void ShowImportLockDialog(gfx::NativeWindow parent,
   base::RecordAction(UserMetricsAction("ImportLockDialogCocoa_Shown"));
 }
 
+void HideImportLockDialog() {
+  // This code path is not needed for Mac, as the user must explicitly close
+  // the sheet before any situation can arise in which we'd need to close it
+  // programmatically. See https://crbug.com/672428 for more details.

[msw, 2017/04/06 00:12:27]

Views should probably instead use a tab or browser-modal dialog to prevent
interaction with the import dialog while the lock dialog is showing? That would
make Views match Cocoa in behavior, and make the weird sub-dialog closing logic
unnecessary.

All you should need for browser window modality is to pass the |parent| to
CreateDialogWidget and override ImportLockDialogView::GetModalType() to return
ui::MODAL_TYPE_WINDOW. For tab modality, see users of
constrained_window::ShowWebModalDialogViews

+}
+
 }  // namespace importer