Try to fix flakiness of fast/repaint/window-resize-background-image*.html

third_party/WebKit/LayoutTests/fast/repaint/resources/window-resize-repaint.js

 var testSizes = [
     { width: 600, height: 500 }, // initial size
     { width: 600, height: 250 }, // height decrease
     { width: 400, height: 250 }, // width decrease
     { width: 400, height: 600 }, // height increase
     { width: 800, height: 600 }  // width increase
 ];
 
 var sizeIndex = 0;
 var repaintRects = "";
@@ skipping 15 matching lines @@
         testRunner.layoutAndPaintAsyncThen(doTest);
     } else if (window.testRunner) {
         testRunner.setCustomTextOutput(repaintRects);
         testRunner.notifyDone();
     }
 }
 
 if (window.testRunner) {
     testRunner.useUnfortunateSynchronousResizeMode();
     testRunner.waitUntilDone();
-    window.resizeTo(testSizes[0].width, testSizes[0].height);
-    testRunner.layoutAndPaintAsyncThen(doTest);
+    onload = function() {
+        window.resizeTo(testSizes[0].width, testSizes[0].height);
+        testRunner.layoutAndPaintAsyncThen(doTest);
+    };
 }