Wait for content_shell to print #READY before starting tests.

This makes it so that we don't include the time to start content_shell
in the timeout for the first test run.

BUG=265088

Review URL: https://codereview.chromium.org/26622004

git-svn-id: svn://svn.chromium.org/blink/trunk@159362 bbb929c8-8fbe-4397-9dbb-9b2b20218538