No bug - remove silly assertion as OwnerDoc() never returns null.

Differential Revision: https://phabricator.services.mozilla.com/D47994

--HG--
extra : moz-landing-system : lando

layout/generic/nsSubDocumentFrame.cpp

@@ -959,7 +959,6 @@ void nsSubDocumentFrame::DestroyFrom(nsIFrame* aDestructRoot,
         ::BeginSwapDocShellsForViews(mInnerView->GetFirstChild());
 
     if (detachedViews && detachedViews->GetFrame()) {
-      MOZ_ASSERT(mContent->OwnerDoc());
       frameloader->SetDetachedSubdocFrame(detachedViews->GetFrame(),
                                           mContent->OwnerDoc());