Re-land Catch up the spec update in CSS Rhythmic Sizing

third_party/WebKit/Source/core/layout/line/RootInlineBox.cpp

Index: third_party/WebKit/Source/core/layout/line/RootInlineBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/line/RootInlineBox.cpp b/third_party/WebKit/Source/core/layout/line/RootInlineBox.cpp
index 6314b189868fc6cab52c10390e96fa3d16362477..5f6d60807a964114156add7f39ee213263353938 100644
--- a/third_party/WebKit/Source/core/layout/line/RootInlineBox.cpp
+++ b/third_party/WebKit/Source/core/layout/line/RootInlineBox.cpp
@@ -221,32 +221,18 @@ void RootInlineBox::childRemoved(InlineBox* box) {
   }
 }
 
-static inline void snapHeight(int& maxAscent,
-                              int& maxDescent,
-                              const ComputedStyle& style) {
-  // If position is 0, add spaces to over/under equally.
-  // https://drafts.csswg.org/css-snap-size/#snap-height
-  int unit = style.snapHeightUnit();
-  ASSERT(unit);
-  int position = style.snapHeightPosition();
-  if (!position) {
-    int space = unit - ((maxAscent + maxDescent) % unit);
-    maxDescent += space / 2;
-    maxAscent += space - space / 2;
+static inline void applyLineHeightStep(uint8_t lineHeightStep,
+                                       int& maxAscent,
+                                       int& maxDescent) {
+  // Round up to the multiple of units, by adding spaces to over/under equally.
+  // https://drafts.csswg.org/css-rhythm/#line-height-step
+  int remainder = (maxAscent + maxDescent) % lineHeightStep;
+  if (!remainder)
     return;
-  }
-
-  // Match the baseline to the specified position.
-  // https://drafts.csswg.org/css-snap-size/#snap-baseline
-  ASSERT(position > 0 && position <= 100);
-  position = position * unit / 100;
-  int spaceOver = position - maxAscent % unit;
-  if (spaceOver < 0) {
-    spaceOver += unit;
-    ASSERT(spaceOver >= 0);
-  }
-  maxAscent += spaceOver;
-  maxDescent += unit - (maxAscent + maxDescent) % unit;
+  DCHECK_GT(remainder, 0);
+  int space = lineHeightStep - remainder;
+  maxDescent += space / 2;
+  maxAscent += space - space / 2;
 }
 
 LayoutUnit RootInlineBox::alignBoxesInBlockDirection(
@@ -278,8 +264,8 @@ LayoutUnit RootInlineBox::alignBoxesInBlockDirection(
     adjustMaxAscentAndDescent(maxAscent, maxDescent, maxPositionTop.toInt(),
                               maxPositionBottom.toInt());
 
-  if (getLineLayoutItem().styleRef().snapHeightUnit())
-    snapHeight(maxAscent, maxDescent, getLineLayoutItem().styleRef());
+  if (uint8_t lineHeightStep = getLineLayoutItem().styleRef().lineHeightStep())
+    applyLineHeightStep(lineHeightStep, maxAscent, maxDescent);
 
   LayoutUnit maxHeight = LayoutUnit(maxAscent + maxDescent);
   LayoutUnit lineTop = heightOfBlock;