Bug 1466727 part 1. Mark nsITreeColumn builtinclass. r=mossop

The only JS implementation seems to be in a test, and we can stop doing that.

browser/components/sessionstore/test/browser_aboutSessionRestore.js

@@ -29,9 +29,11 @@ add_task(async function() {
 
   ok(gBrowser.tabs.length > 1, "we have more than one tab");
 
-  let view = browser.contentDocument.getElementById("tabList").view;
+  let tree = browser.contentDocument.getElementById("tabList");
+  let view = tree.view;
   ok(view.isContainer(0), "first entry is the window");
-  is(view.getCellProperties(1, { id: "title" }), "icon",
+  let titleColumn = tree.columns.title;
+  is(view.getCellProperties(1, titleColumn), "icon",
     "second entry is the tab and has a favicon");
 
   browser.messageManager.loadFrameScript(FRAME_SCRIPT, true);

layout/xul/tree/nsITreeColumns.idl

@@ -7,7 +7,7 @@
 webidl Element;
 webidl TreeColumns;
 
-[scriptable, uuid(ae835ecf-6b32-4660-9b43-8a270df56e02)]
+[scriptable, builtinclass, uuid(ae835ecf-6b32-4660-9b43-8a270df56e02)]
 interface nsITreeColumn : nsISupports
 {
   readonly attribute Element element;