Fix percent height calc for svg docs.

third_party/WebKit/LayoutTests/svg/custom/masking-clipping-hidpi.svg

 <svg xmlns="http://www.w3.org/2000/svg" onload="startTest()">
     <script>
+    if (self.internals)

[pdr., 2017/05/03 15:27:23]

Can you add a minimal test that just covers this bug? I could see someone
rebaselining this without realizing the implications.

+        internals.runtimeFlags.overlayScrollbarsEnabled = true;
+
     function startTest() {
         if (!window.testRunner)
             return;
 
         testRunner.waitUntilDone();
         testRunner.setBackingScaleFactor(2, finishTest);
     }
 
     function finishTest() {
         setTimeout(function () { testRunner.notifyDone(); }, 0);
@@ skipping 30 matching lines @@
     </defs>
     <rect x="0" y="60" width="100" height="100" clip-path="url(#circleClipPath)"/>
 
     <defs>
         <pattern id="Pattern" width="1" height="1">
             <circle cx="25" cy="25" r="20" fill="url(#blackGradient)"/>
         </pattern>
     </defs>
     <rect fill="url(#Pattern)" x="0" y="162" width="200" height="200"/>
 </svg>