Deflake background-image-relative-url-in-iframe.html

third_party/WebKit/LayoutTests/fast/backgrounds/background-image-relative-url-in-iframe.html

Index: third_party/WebKit/LayoutTests/fast/backgrounds/background-image-relative-url-in-iframe.html
diff --git a/third_party/WebKit/LayoutTests/fast/backgrounds/background-image-relative-url-in-iframe.html b/third_party/WebKit/LayoutTests/fast/backgrounds/background-image-relative-url-in-iframe.html
index fadf494c3c627127c2ac49d796b8b8f669c9353a..2d601ec5a54dca8311c9ac9b2009741649dfd1ad 100644
--- a/third_party/WebKit/LayoutTests/fast/backgrounds/background-image-relative-url-in-iframe.html
+++ b/third_party/WebKit/LayoutTests/fast/backgrounds/background-image-relative-url-in-iframe.html
@@ -3,6 +3,7 @@
 <script>
   var iframe = document.getElementById('iframe');
   iframe.contentDocument.body.innerHTML = `
+    <link rel='preload' href='./resources/green-24x24.png' as='image'>

[rune, 2017/04/11 07:43:26]

Will rel=preload guarantee that the resource will be loaded for the first paint
here? If so, lgtm. Otherwise, waiting on a load event would be safer?

[suzyh_UTC10 (ex-contributor), 2017/04/12 05:16:26]

The preload spec says "The preload link element must not delay the load event of
the element's node document." so I guess no, there's no guarantees. Rewritten to
use onload=testRunner.notifyDone instead.

     <style>
       .bg-img {
         background: url('./resources/green-24x24.png') no-repeat;