Convert some pixel tests in fast/canvas into ref tests.

LayoutTests/fast/canvas/font-update-expected.html

 <style>
     @font-face {
         font-family: no-such-font;
         src: url(no-such-file.ttf);
     }
 </style>
 <canvas id="target"></canvas>
 <script>
     canvas = document.getElementById("target");
     ctx = canvas.getContext('2d');
     ctx.font = "100px no-such-font, ahem";
     ctx.fillStyle = "red";
     ctx.fillText("B", 0, 100);
     ctx.fillStyle = "green";
-    canvas.parentNode.removeChild(canvas);
-    if (window.testRunner) {
-        testRunner.dumpAsTextWithPixelResults();
-        testRunner.waitUntilDone();
-        }
-    setTimeout(function()
-    {
-        ctx.fillText("A", 0, 100);
-        document.body.appendChild(canvas);
-        if (window.testRunner)
-            testRunner.notifyDone();
-    }, 50);
+    ctx.fillText("A", 0, 100);
 </script>