There is no "window" global in a web worker context. Use "self" instead.

Fixes #26192

Change-Id: I6c6f3db6c3d3d9ca00a473f8c18b849bc07a0017
Reviewed-on: https://go-review.googlesource.com/122055
Run-TryBot: Richard Musiol <neelance@gmail.com>
Reviewed-by: Richard Musiol <neelance@gmail.com>