Fix a broken WebGL layout test by using testRunner.displayAsyncThen().

LayoutTests/fast/canvas/webgl/draw-webgl-to-canvas-2d.html

Index: LayoutTests/fast/canvas/webgl/draw-webgl-to-canvas-2d.html
diff --git a/LayoutTests/fast/canvas/webgl/draw-webgl-to-canvas-2d.html b/LayoutTests/fast/canvas/webgl/draw-webgl-to-canvas-2d.html
index 4fbcd2287a372bb330706410a5ed2bf2a1418ad8..72aa06fedf1cf7e3cd2ad576f9e8545abba09c93 100644
--- a/LayoutTests/fast/canvas/webgl/draw-webgl-to-canvas-2d.html
+++ b/LayoutTests/fast/canvas/webgl/draw-webgl-to-canvas-2d.html
@@ -86,7 +86,15 @@ function drawWebGLToCanvas2D(ctx2D, canvas3D, isDrawingBufferUndefined) {
         testRunner.notifyDone();
 }
 
-window.onload = function () {
+function asyncTest() {
+    debug("Check for drawing webgl to canvas 2d several frames after drawing webgl contents.")
+    debug("1) when drawingBuffer is preserved.")
+    drawWebGLToCanvas2D(preserve_ctx2D, preserve_canvas3D, false);
+    debug("2) when drawingBuffer is not preserved. It leads to undefined behavior.")
+    drawWebGLToCanvas2D(nonpreserve_ctx2D, nonpreserve_canvas3D, true);
+}
+
+function startTestAfterFirstPaint() {
     // create both canvas 2d and webgl contexts.
     createContexts();
     // prepare webgl contents.
@@ -99,13 +107,16 @@ window.onload = function () {
     debug("2) when drawingBuffer is not preserved.")
     drawWebGLToCanvas2D(nonpreserve_ctx2D, nonpreserve_canvas3D, false);
 
-    setTimeout(function () {
-        debug("Check for drawing webgl to canvas 2d several frames after drawing webgl contents.")
-        debug("1) when drawingBuffer is preserved.")
-        drawWebGLToCanvas2D(preserve_ctx2D, preserve_canvas3D, false);
-        debug("2) when drawingBuffer is not preserved. It leads to undefined behavior.")
-        drawWebGLToCanvas2D(nonpreserve_ctx2D, nonpreserve_canvas3D, true);
-    }, 50);
+    if (window.testRunner) {
+        testRunner.waitUntilDone();
+        testRunner.displayAsyncThen(asyncTest);
+    } else {
+        window.requestAnimationFrame(asyncTest);
+    }
+}
+
+window.onload = function () {
+    window.requestAnimationFrame(startTestAfterFirstPaint);
 }
 
 </script>