Bug 1240225 - Make test not racey. r=emilio

Depending on the chunking and timing of the HTML parser, we may end up
firing onload on the image before the script tag is evaluated, leading
to an undefined onLoad (which is the intermittent failure in the test).

MozReview-Commit-ID: 78OAZan1xbC

dom/html/test/test_bug1166138.html

@@ -16,10 +16,6 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1166138
   <div id="content" style="display: none">
   </div>
 
-  <img srcset="file_bug1166138_1x.png 1x, file_bug1166138_2x.png 2x"
-       src="file_bug1166138_def.png"
-       onload="onLoad()">
-
   <script type="application/javascript">
     var img1x = "http://mochi.test:8888/tests/dom/html/test/file_bug1166138_1x.png";
     var img2x = "http://mochi.test:8888/tests/dom/html/test/file_bug1166138_2x.png";
@@ -126,5 +122,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1166138
       done = true;
     });
   </script>
+
+  <img srcset="file_bug1166138_1x.png 1x, file_bug1166138_2x.png 2x"
+       src="file_bug1166138_def.png"
+       onload="onLoad()">
+
 </body>
 </html>