cmd/internal/browser: wait 3 seconds for non-zero exit codes

cmd/internal/browser: wait 3 seconds for non-zero exit codes

Wait a short period between trying commands. Many commands
will return a non-zero exit code if the browser couldn't be launched.

For example, google-chrome returns quickly with a non-zero
exit code in a headless environment.

Updates #19131.

Change-Id: I0ae5356dd4447969d9e216615449cead7a8fd5c9
Reviewed-on: https://go-review.googlesource.com/37391
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>