Bug 1666827 - Mark browsing contexts where we host static documents as "printing". r=smaug

This prevents e.g. navigating the print preview document.

I can try to come up with a test for this, though testing for something
not happening is always a bit more annoying... :)

Depends on D91438

Differential Revision: https://phabricator.services.mozilla.com/D91439

dom/base/nsGlobalWindowOuter.cpp

@@ -5357,6 +5357,8 @@ Nullable<WindowProxyHolder> nsGlobalWindowOuter::Print(
       aError.ThrowNotAllowedError("No browsing context");
       return nullptr;
     }
+
+    Unused << bc->Top()->SetIsPrinting(true);
     nsCOMPtr<nsIDocShell> cloneDocShell = bc->GetDocShell();
     MOZ_DIAGNOSTIC_ASSERT(cloneDocShell);
     cloneDocShell->GetContentViewer(getter_AddRefs(cv));