Bug 1660665, hide subdialog if user chooses to print using system dialog. r=jwatt,Gijs,mstriemer

When the user chooses to print using the system dialog, we should hide the print
UI. We are choosing to hide the dialog stack instead of closing the dialog because
the print preview browser still needs to be available if the user tries to print.
We close the window if the user cancels the system dialog or once we receive the
promise from PrintUtils.printWindow.

Differential Revision: https://phabricator.services.mozilla.com/D88096

browser/base/content/browser.js

@@ -9045,6 +9045,10 @@ class TabDialogBox {
     }
     return browser;
   }
+
+  getManager() {
+    return this._dialogManager;
+  }
 }
 
 TabDialogBox.prototype.QueryInterface = ChromeUtils.generateQI([