Remove unnecessary anonymous blocks when splitting anonymous boxes

third_party/WebKit/Source/core/layout/LayoutBox.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.cpp b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
index 65fcc59a155771b05adca84f275a5b7209287676..f7dcd81787bc6b285ae6c2f231cf9764dcd95327 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -4372,6 +4372,17 @@ static void markBoxForRelayoutAfterSplit(LayoutBox* box)
     box->setNeedsLayoutAndPrefWidthsRecalcAndFullPaintInvalidation(LayoutInvalidationReason::AnonymousBlockChange);
 }
 
+static void collapseLoneAnonymousBlockChild(LayoutObject* child)
+{
+    ASSERT(child);
+    if (!child->isAnonymousBlock())
+        return;
+    LayoutObject* parent = child->parent();
+    if (!parent->isLayoutBlock())
+        return;
+    LayoutBlock::collapseAnonymousBlockChild(toLayoutBlock(parent), toLayoutBlock(child));
+}
+
 LayoutObject* LayoutBox::splitAnonymousBoxesAroundChild(LayoutObject* beforeChild)
 {
     bool didSplitParentAnonymousBoxes = false;
@@ -4393,6 +4404,13 @@ LayoutObject* LayoutBox::splitAnonymousBoxesAroundChild(LayoutObject* beforeChil
             parentBox->virtualChildren()->insertChildNode(parentBox, postBox, boxToSplit->nextSibling());
             boxToSplit->moveChildrenTo(postBox, beforeChild, 0, true);
 
+            LayoutObject* child = postBox->slowFirstChild();
+            if (child && !child->nextSibling())
+                collapseLoneAnonymousBlockChild(child);
+            child = boxToSplit->slowFirstChild();

[mstensho (USE GERRIT), 2016/02/01 12:25:36]

Could ASSERT(child) here (and above) instead of "if (child)". Why split if there
are no children?

[rhogan, 2016/02/02 21:38:25]

True. I think we should ASSERT and null check though. 

+            if (child && !child->previousSibling())
+                collapseLoneAnonymousBlockChild(child);
+
             markBoxForRelayoutAfterSplit(boxToSplit);
             markBoxForRelayoutAfterSplit(postBox);