Implement convertToBlob API for OffscreenCanvas

[jdm]

WebIDL: https://html.spec.whatwg.org/multipage/canvas.html#the-offscreencanvas-interface
Spec: https://html.spec.whatwg.org/multipage/canvas.html#dom-offscreencanvas-converttoblob
Code:

• components/script/dom/webidls/OffscreenCanvas.webidl
• components/script/dom/offscreencanvas.rs

Tests: ./mach test-wpt tests/wpt/web-platform-tests/offscreen-canvas/convert-to-blob/

[gterzian]

This might be best done after #24123, since that PR implies some fairly significant changes to Blob(a bunch of reshuffling really).

[gterzian]

Also, we might want to first implement Transferable for OffscreenCanvas, since that might imply some reshuffling in the implementation of OffscreenCanvas(I assume something similar as the split between MessagePort and MessagePortImpl being done in #23637), and such as change could signifanctly affect the implementation of convertToBlob. See #24276

[jdm]

I doubt that those will have a significant impact on the implementation here - this task is focused on getting the pixels from the canvas and stashing them in a blob, so the implementation details of either the canvas or blob shouldn't really affect it.