Race condition in SeleniumComJSRunner with onReceive.

[nicolasstucki]

It seams that in some cases onReceive is called before init. Adding a buffer for incoming messages before init is called should solve this issue.

https://github.com/scala-js/scala-js-env-selenium/blob/master/seleniumJSEnv/src/main/scala/org/scalajs/jsenv/selenium/SeleniumComJSRunner.scala#L106