tour: kill running code snippets when changing page
tour: kill running code snippets when changing page

When a new page is loaded, the backend is told to kill still running code snippets,
known to the frontend at this point.

Fixes golang/go#16418

Change-Id: Ie4c246b6bf9f566ef7523e35b44395a1abfd4e41
Reviewed-on: https://go-review.googlesource.com/98675
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

Go-zh/tour,Go-zh/tour,Go-zh/tour,Go-zh/tour