code: function(tabId, url) {
          var entry = tabContexts.get(tabId);
          if ( entry === undefined ) {
              entry = new TabContext(tabId);
              entry.autodestroy();
          }
          entry.push(url);
          mostRecentRootDocURL = url;
          mostRecentRootDocURLTimestamp = Date.now();
          return entry;
      }
docs: These are to be used for the API of the tab context manager.