Bug 1821303 - Add getter for activeSessionHistoryEntry to BC r=peterv

Add activeSessionHistoryEntry attribute to CanonicalBrowsingContext to
expose the existing implementation of
CanonicalBrowsingContext::GetActiveSessionHistoryEntry.

Differential Revision: https://phabricator.services.mozilla.com/D173235

dom/bindings/Bindings.conf

@@ -2107,6 +2107,7 @@ addExternalIface('URI', nativeType='nsIURI', headerFile='nsIURI.h',
                  notflattened=True)
 addExternalIface('XULCommandDispatcher', notflattened=True)
 addExternalIface('nsISHistory', nativeType='nsISHistory', notflattened=True)
+addExternalIface('nsISHEntry', nativeType='nsISHEntry', notflattened=True)
 addExternalIface('ReferrerInfo', nativeType='nsIReferrerInfo')
 addExternalIface('nsIPermissionDelegateHandler',
                  nativeType='nsIPermissionDelegateHandler',

dom/chrome-webidl/BrowsingContext.webidl

@@ -6,6 +6,7 @@
 interface URI;
 interface nsIDocShell;
 interface nsISecureBrowserUI;
+interface nsISHEntry;
 interface nsIPrintSettings;
 interface nsIWebProgress;
 
@@ -345,6 +346,7 @@ interface CanonicalBrowsingContext : BrowsingContext {
   undefined stop(unsigned long aStopFlags);
 
   readonly attribute nsISHistory? sessionHistory;
+  readonly attribute nsISHEntry? activeSessionHistoryEntry;
 
   readonly attribute MediaController? mediaController;