Sweep for menu commands referencing stale windows.

Refs greasemonkey#2067

content/menucommander.js

@@ -16,6 +16,29 @@ GM_MenuCommander.initialize = function() {
       GM_MenuCommander.clearMenuCommands);
   messageManager.addMessageListener('greasemonkey:toggle-menu-commands',
       GM_MenuCommander.toggleMenuCommands);
+
+  setInterval(GM_MenuCommander.sweep, 66000);
+};
+
+GM_MenuCommander.sweep = function() {
+  for (var windowId in GM_MenuCommander.menuCommands) {
+    var commands = GM_MenuCommander.menuCommands[windowId];
+
+    // Rather than mutate the (commands) array in the loop, which is failure
+    // prone, just track whether it is "empty".  All commands should go
+    // stale together, or not.  If empty, we can safely mutate the object
+    // (menuCommands) to clean it out, even inside its loop.
+    var empty = true;
+    for (var i in commands) {
+      var b = commands[i].browser.get();
+      if (b) empty = false;
+    }
+
+    if (empty) {
+      GM_MenuCommander.menuCommands[windowId] = null;
+      delete GM_MenuCommander.menuCommands[windowId];
+    }
+  }
 };
 
 GM_MenuCommander.menuCommandRegistered = function(aMessage) {