Improve gpu pixel test for OffscreenCanvas

content/test/data/gpu/pixel_offscreenCanvas_webgl_commit_worker.html

Index: content/test/data/gpu/pixel_offscreenCanvas_webgl_commit_worker.html
diff --git a/content/test/data/gpu/pixel_offscreenCanvas_webgl_commit_worker.html b/content/test/data/gpu/pixel_offscreenCanvas_webgl_commit_worker.html
index 292500ea9b39392e52a27e2ab9aa960d5fb866c0..77cb4c934e8c22dc80ff3aaf8dc2df5195dc45dd 100644
--- a/content/test/data/gpu/pixel_offscreenCanvas_webgl_commit_worker.html
+++ b/content/test/data/gpu/pixel_offscreenCanvas_webgl_commit_worker.html
@@ -2,7 +2,7 @@
 
 <!-- READ BEFORE UPDATING:
 If this test is updated make sure to increment the "revision" value of the
-associated test in content/test/gpu/page_sets/pixel_tests.py. This will ensure
+associated test in content/test/gpu/gpu_tests/pixel_test_pages.py. This will ensure
 that the baseline images are regenerated on the next run.
 -->
 
@@ -84,10 +84,10 @@ function draw()
   var canvas = document.getElementById("c");
   var offscreenCanvas = canvas.transferControlToOffscreen();
   var worker = makeWorker(document.getElementById("myWorker").textContent);
-  worker.postMessage(offscreenCanvas, [offscreenCanvas]);
   worker.onmessage = function (e) {
     waitForFinish();
   }
+  worker.postMessage(offscreenCanvas, [offscreenCanvas]);
 }
 
 function waitForFinish()