Fix firing a chrome event when there are no listeners.
Fix firing a chrome event when there are no listeners.

It threw an exception from .length not being defined.

MobileChromeApps/mobile-chrome-apps,xiaoyanit/mobile-chrome-apps,chirilo/mobile-chrome-apps,hgl888/mobile-chrome-apps,guozanhua/mobile-chrome-apps,MobileChromeApps/mobile-chrome-apps,hgl888/mobile-chrome-apps,chirilo/mobile-chrome-apps,xiaoyanit/mobile-chrome-apps,liqingzhu/mobile-chrome-apps,hgl888/mobile-chrome-apps,hgl888/mobile-chrome-apps,xiaoyanit/mobile-chrome-apps,MobileChromeApps/mobile-chrome-apps,liqingzhu/mobile-chrome-apps,hgl888/mobile-chrome-apps,wudkj/mobile-chrome-apps,xiaoyanit/mobile-chrome-apps,guozanhua/mobile-chrome-apps,wudkj/mobile-chrome-apps,hgl888/mobile-chrome-apps,hgl888/mobile-chrome-apps,liqingzhu/mobile-chrome-apps,guozanhua/mobile-chrome-apps,wudkj/mobile-chrome-apps,guozanhua/mobile-chrome-apps,wudkj/mobile-chrome-apps,chirilo/mobile-chrome-apps,chirilo/mobile-chrome-apps,MobileChromeApps/mobile-chrome-apps,liqingzhu/mobile-chrome-apps