Bug 1463291: Move docShell getter from Document to Window. r=bz

DocShells are associated with outer DOM Windows, rather than Documents, so
having the getter on the document is a bit odd to begin with. But it's also
considerably less convenient, since most of the times when we want a docShell
from JS, we're dealing most directly with a window, and have to detour through
the document to get it.

MozReview-Commit-ID: LUj1H9nG3QL

browser/components/nsBrowserContentHandler.js

@@ -670,7 +670,7 @@ function handURIToExistingBrowser(uri, location, cmdLine, forcePrivate, triggeri
     return;
   }
 
-  var bwin = navWin.document.docShell.rootTreeItem.domWindow;
+  var bwin = navWin.docShell.rootTreeItem.domWindow;
   bwin.openURI(uri, null, location,
                Ci.nsIBrowserDOMWindow.OPEN_EXTERNAL, triggeringPrincipal);
 }