Fix percent height calc for svg docs.

Previously, percent-height svg elements in a pure-svg doc would get
their size based on the LayoutView's logical height from the previous
layout pass.  In practice, this didn't show up much because FrameView
always does at least one extra initial layout pass to add and then
subtract scrollbars.  The extra layout pass would cause the percent
height descendants to get the right size.

Setting overlay scrollbars on the FrameView eliminates the extra
layout passes and allows the bug to manifest.

Review-Url: https://codereview.chromium.org/2835153002
Cr-Commit-Position: refs/heads/master@{#469327}

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

+rect.offsetHeight = 600

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>

third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.cpp

@@ -26,6 +26,7 @@
 #include "core/frame/LocalFrame.h"
 #include "core/layout/HitTestResult.h"
 #include "core/layout/LayoutAnalyzer.h"
+#include "core/layout/LayoutView.h"
 #include "core/layout/api/LayoutPartItem.h"
 #include "core/layout/svg/LayoutSVGText.h"
 #include "core/layout/svg/SVGLayoutSupport.h"
@@ -134,6 +135,13 @@ LayoutUnit LayoutSVGRoot::ComputeReplacedLogicalHeight(
     return ContainingBlock()->AvailableLogicalHeight(
         kIncludeMarginBorderPadding);
 
+  const Length& logical_height = Style()->LogicalHeight();
+  if (IsDocumentElement() && logical_height.IsPercentOrCalc()) {
+    return ValueForLength(
+        logical_height,
+        GetDocument().GetLayoutView()->ViewLogicalHeightForPercentages());
+  }
+
   return LayoutReplaced::ComputeReplacedLogicalHeight(estimated_used_width);
 }
 

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5

@@ -696,6 +696,7 @@
     },
     {
       name: "OverlayScrollbars",
+      settable_from_internals: true,
     },
     {
       name: "PagePopup",