[web] move checking for desktop Safari to constant

Summary:
I misunderstood @tomek's suggestion from here: https://phab.comm.dev/D8124?id=27491#inline-52847

Moving not only function result to const but entire logic as this will not change during app lifetime (this is not true for tests but this is handled separately)

Test Plan:

1. Check if this const is truthy for Safari
2. Check if this const is falsy for other browser (like chrome)

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8226