Fix TCP server port race for Chrome (#47229)
Before this fix WptRunner searched for a free TCP port and then created
a ChromeDriver instance that would have to listen on this port.
This created a race with other system processes that could steal the
port while the ChromeDriver instance was starting.
With this commit WptRunner requests ChromeDriver to find a free port and
to start listening on it. ChromeDriver communicates the server port
number back to WptRunner via stdout. This prevents any competing process
from stealing the TCP port.
Bug: crbug.com/42323836