Fix 'text-underline-position: under' to use em height ascent/descent

third_party/WebKit/Source/core/layout/line/InlineBox.h

Index: third_party/WebKit/Source/core/layout/line/InlineBox.h
diff --git a/third_party/WebKit/Source/core/layout/line/InlineBox.h b/third_party/WebKit/Source/core/layout/line/InlineBox.h
index bd77dee07414a993e7b32950273ce0f408755675..105bf7ad92e203b91c132e8ac30bc9041cd081fb 100644
--- a/third_party/WebKit/Source/core/layout/line/InlineBox.h
+++ b/third_party/WebKit/Source/core/layout/line/InlineBox.h
@@ -39,6 +39,27 @@ class RootInlineBox;
 
 enum MarkLineBoxes { kMarkLineBoxesDirty, kDontMarkLineBoxes };
 
+enum class LineVerticalPositionType {
+  // TextTop and TextBottom are the top/bottom of the content area.
+  // This is where 'vertical-align: text-top/text-bottom' aligns to.
+  // This is explicitly undefined in CSS2.
+  // https://drafts.csswg.org/css2/visudet.html#inline-non-replaced
+  TextTop,
+  TextBottom,
+  // Em height as being discussed in Font Metrics API.
+  // https://drafts.css-houdini.org/font-metrics-api-1/#fontmetrics
+  TopOfEmHeight,
+  BottomOfEmHeight
+};
+
+// Returns whether the position type is CSS "line-over"; i.e., ascender side
+// or "top" side of a line box.
+// https://drafts.csswg.org/css-writing-modes-3/#line-over
+static inline bool IsLineOverSide(LineVerticalPositionType type) {
+  return type == LineVerticalPositionType::TextTop ||
+         type == LineVerticalPositionType::TopOfEmHeight;
+}
+
 // InlineBox represents a rectangle that occurs on a line.  It corresponds to
 // some LayoutObject (i.e., it represents a portion of that LayoutObject).
 class CORE_EXPORT InlineBox : public DisplayItemClient {