DebuggerDispatchTable = function() {
  /**
   * Total number of attached debugger instances.
   * @type {number}
   * @private
   */
  this.count_ = 0;

  /**
   * Registered tabs, mapped by tab ID.
   * @type {!Object.<number, !Debugger>}
   * @private
   */
  this.tabs_ = {};

  /**
   * Registered event handlers.
   * @type {!Object}
   * @private
   */
  this.eventHandlers_ = {
    onEvent: this.onEvent_.bind(this),
    onDetach: this.onDetach_.bind(this)
  };
}
Shared dispatch table for debugger events.
Modern Chromes only allow a single event handler to be registered on
chrome.debugger, so we have to map it back to the right place here.
@constructor