Bug 844530 - Paper over error in BrowserElementPromptService.jsm on B2G. r=khuey

The error we're fixing is

  E/GeckoConsole(  397): [JavaScript Error: "NS_ERROR_FACTORY_NOT_REGISTERED: Component returned failure code: 0x80040154 (NS_ERROR_FACTORY_NOT_REGISTERED)
  [nsIComponentManager.getClassObject]" {file: "resource://gre/modules/BrowserElementPromptService.jsm" line: 572}]

For some reason, we're running code before the vanilla prompt service is
instantiated.  We don't know why this is happening, but this patch works
around the problem on B2G, which is the only place where we ship this
code.