Use a stronger atomic guarantee when cancelling tasks

[gterzian]

In the context of #22507

Note that the "other side" of these operations is at

servo/components/script/task.rs

Line 102 in 0b61cfc

.map_or(false, |cancelled| cancelled.load(Ordering::SeqCst))

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #___ (GitHub issue number if applicable)

• There are tests for these changes OR
• These changes do not require tests because ___

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/dom/window.rs
• @KiChjang: components/script/dom/window.rs

[highfive]

Warning

• These commits modify script code, but no tests are modified. Please consider adding a test!

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit 58259c1 has been approved by jdm

[bors-servo]

⌛ Testing commit 58259c1 with merge 1cd78e7...

[bors-servo]

☀️ Test successful - status-taskcluster
Approved by: jdm
Pushing 1cd78e7 to master...