Fix percent height calc for svg docs.

third_party/WebKit/LayoutTests/svg/custom/percent-height.svg

+<svg xmlns="http://www.w3.org/2000/svg" onload="startTest()">
+<script>
+if (self.internals)
+  internals.runtimeFlags.overlayScrollbarsEnabled = true;
+
+function startTest() {
+  if (!window.testRunner)
+    return;
+
+  testRunner.dumpAsText();
+  testRunner.waitUntilDone();
+  testRunner.setBackingScaleFactor(2, finishTest);
+  var rect = document.getElementById('rect');
+  var result = document.getElementById('result');
+  result.innerHTML = 'rect.offsetHeight = ' + rect.getBoundingClientRect().height;
+}
+
+function finishTest() {
+  setTimeout(function () { testRunner.notifyDone(); }, 0);
+}
+</script>
+
+<mask id='mask'>
+  <rect id='rect' width='100%' height='100%' fill='white'/>
+</mask>
+<text id='result'></text>
+</svg>