Fix percent height calc for svg docs.

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

Index: third_party/WebKit/LayoutTests/svg/custom/masking-clipping-hidpi.svg
diff --git a/third_party/WebKit/LayoutTests/svg/custom/masking-clipping-hidpi.svg b/third_party/WebKit/LayoutTests/svg/custom/masking-clipping-hidpi.svg
index a179832fb6d420b540d3e964c633b22ddd27411b..f6ea33f82c352f24bd676340345ab768e01c69e8 100644
--- a/third_party/WebKit/LayoutTests/svg/custom/masking-clipping-hidpi.svg
+++ b/third_party/WebKit/LayoutTests/svg/custom/masking-clipping-hidpi.svg
@@ -1,5 +1,8 @@
 <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;