Bug 1552162. Fix unified build bustage in nsPrintObject.cpp. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D31446

--HG--
extra : rebase_source : b4ef894cfd919455788c82392d385a41b27c77be
extra : amend_source : d5c3542c4cb15185e65bf85872231a6e8e3055cd

layout/printing/nsPrintObject.cpp

@@ -22,7 +22,9 @@
 #include "mozilla/dom/Document.h"
 #include "mozilla/dom/Element.h"
 
+using mozilla::PresShell;
 using mozilla::dom::BrowsingContext;
+using mozilla::dom::Document;
 using mozilla::dom::Element;
 
 //---------------------------------------------------