Force glyph overflow calculation

LayoutTests/fast/dom/Element/getBoundingClientRect.html

 <script src="../../../resources/js-test.js"></script>
 <style>
     body {
         font: 16px Ahem;
     }
 
     #base {
         width: 300px;
         height: 100px;
     }
@@ skipping 115 matching lines @@
         document.documentElement.appendChild(bbox);
     }
 
     var fail = undefined;
 
     var expectedResults = [
         /*1*/  { left: 8, top: 40, width: 300, height: 100 },
         /*2*/  { left: 8, top: 188, width: 320, height: 120 },
         /*3*/  { left: 18, top: 356, width: 300, height: 100 },
         /*4*/  { left: 37.289, top: 483.289, width: 141.421, height: 141.421 },
+        // On Mac, this fails because of crbug.com/498030.
         /*5*/  { left: 8, top: 652, width: 300, height: 368 },
         /*6*/  { left: fail, top: fail, width: fail, height: fail },
         /*7*/  { left: 8, top: 1484, width: 288, height: 464 },
         /*8*/  { left: 8, top: 1996, width: 300, height: 74 },
         /*9*/  { left: 8, top: 2134, width: 300, height: 90 },
         /*10*/ { left: 8, top: 2274, width: 300, height: 34 },
         /*11*/ { left: 10, top: 2396, width: 147, height: 34 },
         /*12*/ { left: 8, top: 2532, width: 100, height: 100 },
         /*13*/ { left: 8, top: 2783, width: 260, height: 16 },
         /*14*/ { left: 8, top: 2863, width: 300, height: 232 },
@@ skipping 17 matching lines @@
 
     var tests = document.getElementsByClassName("testBox");
     for (var i = 0; i < tests.length; ++i)
         test(i + 1, tests[i]);
 
     if (window.testRunner) {
         var area = document.getElementById('testArea');
         area.parentNode.removeChild(area);
     }
 </script>