[mac] LayoutTest svg/as-image/svg-image-with-data-uri-background.html is a flaky image failure

[Ryan Haddad]

LayoutTest svg/as-image/svg-image-with-data-uri-background.html is a flaky image failure

https://build.webkit.org/results/Apple%20El%20Capitan%20Debug%20WK1%20(Tests)/r217720%20(1605)/results.html

https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=svg%2Fas-image%2Fsvg-image-with-data-uri-background.html

Direct link to images that show a red square where a green one was expected: https://build.webkit.org/results/Apple%20El%20Capitan%20Debug%20WK1%20(Tests)/r217720%20(1605)/svg/as-image/svg-image-with-data-uri-background-diffs.html

[Alexey Proskuryakov]

This doesn't look good. It sounds like a WebKit bug if we don't wait for SVG subresources before dispatching the load event.

      // window load event fires when the the page and its resources are loaded.
      // However it does not wait till the sub-resources are loaded. And this is
      // why we need to wait a little bit before we call notifyDone().
      window.addEventListener("load", function() {
        setTimeout(function () {
          testRunner.notifyDone();
        }, 200);
      });

[Matt Lewis]

This is still happening on Mac Debug WK1 as of this build:

https://build.webkit.org/results/Apple%20El%20Capitan%20Debug%20WK1%20(Tests)/r221372%20(3305)/results.html

https://build.webkit.org/builders/Apple%20El%20Capitan%20Debug%20WK1%20(Tests)/builds/3305

The failure is still the same as well.

https://build.webkit.org/results/Apple%20El%20Capitan%20Debug%20WK1%20(Tests)/r221372%20(3305)/svg/as-image/svg-image-with-data-uri-background-diffs.html

[Matt Lewis]

Marked test as flaky:
https://trac.webkit.org/changeset/221638/webkit