Bug 1718204 - Ensure that there's a content viewer in the docshell we're cloning a document into. r=jfkthame,layout-reviewers

The code below assumes that, and bug 1666247 was hitting assertions
because of that, see D116877.

Differential Revision: https://phabricator.services.mozilla.com/D118810

dom/base/nsGlobalWindowOuter.cpp

@@ -5303,6 +5303,8 @@ Nullable<WindowProxyHolder> nsGlobalWindowOuter::Print(
     MOZ_DIAGNOSTIC_ASSERT(cv);
   } else {
     if (aDocShellToCloneInto) {
+      // Ensure the content viewer is created if needed.
+      Unused << aDocShellToCloneInto->GetDocument();
       bc = aDocShellToCloneInto->GetBrowsingContext();
     } else {
       AutoNoJSAPI nojsapi;