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

toolkit/components/extensions/extension-process-script.js

@@ -89,7 +89,7 @@ var contentScripts = new DefaultWeakMap(matcher => {
 });
 
 function getMessageManager(window) {
-  let docShell = window.document.docShell.QueryInterface(Ci.nsIInterfaceRequestor);
+  let docShell = window.docShell.QueryInterface(Ci.nsIInterfaceRequestor);
   try {
     return docShell.getInterface(Ci.nsIContentFrameMessageManager);
   } catch (e) {