Do not block waiting for chromedriver to start up

[WeizhongX]

This would save approximately 0.5 second on each test runner restarts. And this should not cause a problem as another connection to the webdriver will be made to init the session, which also happens during initialization stage.

https://github.com/web-platform-tests/wpt/blob/master/tools/wptrunner/wptrunner/testrunner.py#L93