Bug 1756728 part 1: Make it possible to retrieve a BrowsingContext from a DocAccessibleParent. r=morgan

Previously, even for remote in-process iframes, it was only possible to retrieve the top level BrowsingContext for the remote process by getting the managing BrowserParent.
This makes it possible to get the correct BrowsingContext even for in-process iframes.

Differential Revision: https://phabricator.services.mozilla.com/D147716

accessible/base/NotificationController.cpp

@@ -965,7 +965,9 @@ void NotificationController::WillRefresh(mozilla::TimeStamp aTime) {
           do_GetInterface(mDocument->DocumentNode()->GetDocShell());
       if (browserChild) {
         static_cast<BrowserChild*>(browserChild.get())
-            ->SendPDocAccessibleConstructor(ipcDoc, parentIPCDoc, id, 0, 0);
+            ->SendPDocAccessibleConstructor(
+                ipcDoc, parentIPCDoc, id,
+                childDoc->DocumentNode()->GetBrowsingContext(), 0, 0);
         ipcDoc->SendPDocAccessiblePlatformExtConstructor();
       }
 #endif