gistfile1.js

function makeTestURL({ body }) {
  const url = new URL(defaultURL);
  url.searchParams.set("Content-Type", "text/html; charset=utf-8");
  url.searchParams.set("body", encodeURIComponent(body));
  return url.href;
}

function getIconColor(window, icon) {
  return new Promise(resolve => {
    const canvas = window.document.createElement('canvas');
    const ctx = canvas.getContext("2d");

// The line above causes
//
//5 INFO TEST-UNEXPECTED-FAIL | dom/manifest/test/browser_ManifestIcons_browserFetchIcon.js | Uncaught exception - at chrome://mochitests/content/browser/dom/manifest/test/browser_ManifestIcons_browserFetchIcon.js:35 - Error: cross-process JS call failed
//Stack trace:
//    getIconColor/<@chrome://mochitests/content/browser/dom/manifest/test/browser_ManifestIcons_browserFetchIcon.js:35:17

    const image = new Image();
    image.onload = function() {
      ctx.drawImage(image, 0, 0);
      resolve(ctx.getImageData(0, 0, 1, 1).data);
    };
    image.src = icon;
  });
}

add_task(function*() {
  const runningTests = tests
    .map(test => ({gBrowser, test, url: makeTestURL(test)}))
    .map(
      tabOptions => BrowserTestUtils.withNewTab(tabOptions, function*(browser) {
        const manifest = yield ManifestObtainer.browserObtainManifest(browser);
        const icon = yield ManifestIcons.browserFetchIcon(browser, manifest, 100);
        const data = yield getIconColor(browser.contentWindow, icon);
        ok(true, 'yay');
      })
    );
  yield Promise.all(runningTests);
});