Extend context loss/restoration test coverage to desynchronized canvas

Desynchronize canvases use a resource dispatcher to push frames and
since the dispatcher is descarded on context loss [1], it's worth adding
test coverage for this.

Note that the frame dispatcher is never recreated after the context is
restored, so the canvas doesn't fully recover from the context loss. The
canvas can be read back though, so the tests still works and we can at
least validate that the context state transitions work properly.

Also note that the OffscreenCanvas doesn't do much with the
`desynchronized` flag, but the test coverage is still useful given that
this flag is listed as supported in MDN [1] and the specification [2].

[1] https://crsrc.org/c/third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.cc;l=290;drc=8abea14deda089834ba142a35e8342014812df55
[2] https://developer.mozilla.org/en-US/docs/Web/API/OffscreenCanvas/getContext
[3] https://html.spec.whatwg.org/multipage/canvas.html#canvas-setting-init-bitmap

Change-Id: I300d1f2827406120896cc7dc38794cb450ea3ed5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7770647
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1617254}

Author: graveljp

html/canvas/resources/canvas-promise-test.js

@@ -144,7 +144,13 @@ function runCanvasTestsInWorker({dependencies = []} = {}) {
     const dependencyScripts =
        await Promise.all(allDeps.map(dep => fetch(dep).then(r => r.text())));
     const canvasTests = currentScript.textContent;
-    const allScripts = dependencyScripts.concat([canvasTests, 'done();']);
+    const allScripts = [
+      // Forward `location.search` to the worker so that it could run the right
+      // test variants. `location.search` is read-only in workers, so the whole
+      // object has to be replaced.
+      `var location = {search: '${self.location.search}'};`,
+    ].concat(dependencyScripts)
+     .concat([canvasTests, 'done();']);
 
     const workerBlob = new Blob(allScripts);
     const worker = new Worker(URL.createObjectURL(workerBlob));