Bug 1399429 - Properly determine if the content window is private for the contextMenu. r=Felipe

MozReview-Commit-ID: Ipo3Rcs1TZ8

--HG--
extra : rebase_source : ac17083a7b3d3d12f39c797c8e073b37c0c1c770

browser/modules/ContextMenu.jsm

@@ -659,7 +659,7 @@ class ContextMenu {
       contentType: context.target.ownerDocument.contentType,
 
       // used for nsContextMenu.saveLink
-      isPrivate: context.target.ownerDocument.isPrivate,
+      isPrivate: PrivateBrowsingUtils.isBrowserPrivate(context.target.ownerDocument),
     };
 
     // used for nsContextMenu.initMediaPlayerItems