Fix flaky unanimated-style.html

animations/unanimated-style.html was using setTimout(0) to wait for the
initial style calculations
before starting the animation. This isn't a reliable method.

We now force style recalc explicitly by reading an element's offsetTop.

BUG=770362

Change-Id: Icf518b3a43b51d854de81cc2fab371922e271e16
Reviewed-on: https://chromium-review.googlesource.com/699974Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506497}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3713,7 +3713,6 @@ crbug.com/767469 virtual/mojo-loading/http/tests/navigation/start-load-during-pr
 crbug.com/767892 [ Win Linux Mac ] virtual/mojo-loading/http/tests/devtools/startup/database-open.html [ Pass Timeout ]
 
 # Sheriff failures 2017-09-29
-crbug.com/770362 [ Linux Debug ] virtual/threaded/animations/unanimated-style.html [ Pass Timeout ]
 
 # Sheriff failures 2017-10-02
 crbug.com/770971 [ Win7 ] fast/forms/suggested-value.html [ Pass Failure ]

third_party/WebKit/LayoutTests/animations/unanimated-style.html

@@ -15,7 +15,7 @@
             animation: move 1s linear;
             left: 400px;
         }
-        
+
         @keyframes move {
             from {
             }
@@ -23,7 +23,7 @@
                 left: 500px;
             }
         }
-        
+
         #result {
             margin-top: 130px;
         }
@@ -31,23 +31,22 @@
     <script src="resources/animation-test-helpers.js" type="text/javascript" charset="utf-8"></script>
     <script type="text/javascript" charset="utf-8">
 
-    if (window.testRunner)
+      if (window.testRunner)
         testRunner.waitUntilDone();
 
       const expectedValues = [
         // [time, element-id, property, expected-value, tolerance]
-        [0.5, "test", "left", 450, 8],
+        [0.5, "test", "left", 450, 0],
       ];
 
       function setupTest()
       {
-        document.getElementById('test').className = 'animating box';
+        test.offsetTop; // force style recalc
+        test.className = 'animating box';
         runAnimationTest(expectedValues);
       }
-      
-      window.addEventListener('load', function() {
-          window.setTimeout(setupTest, 0);
-      }, false);
+
+      window.addEventListener('load', setupTest, false);
 
     </script>
 </head>