Restart chromedriver when webview closes

When the window ChromeDriver has a reference to is lost, subsequent commands will fail. So test that it is there (by sending the simplest non-destructive command possible, `/url`) and, if not, reinitialize the session.

Fixes #3775.
Do you think it's worth breaking this up into a couple functions?  It might be easier to read and would be easier to test if we ever wanted to write unit tests.