[mv3] Use globalThis instead of window

[emaxx-google]

Switch a couple of places that use the "window" object to use "globalThis" instead. The latter is a better equivalent because it works in Manifest V3 Service Workers as well (in a Service Worker, "self" should be used instead of "window"; "globalThis" is an alias to either of those).

This contributes to the effort tracked by #1129.

[github-actions[bot]]

Continuous Integration: All tests passed. C/C++ test coverage: 73.36% lines.

[emaxx-google]

Thanks for the reviews!