Warm up image cache for data URI used in tests

The data URI for image tests were added in:
https://codereview.chromium.org/1754213004/
It relies on sync loading of the images, which is no longer the case
after https://codereview.chromium.org/2173003002/.

R=nyquist@chromium.org

Review URL: https://codereview.chromium.org/2179593002 .
diff --git a/war/test.html b/war/test.html
index 6a3fa83..a0c8b3f 100644
--- a/war/test.html
+++ b/war/test.html
@@ -2,5 +2,8 @@
 <head>
   <script src="domdistillerjstest/domdistillerjstest.nocache.js"></script>
 </head>
+<body>
+<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==">
 hello body
+</body>
 </html>