Avoid layout when list color changes

Source/core/rendering/RenderListItem.cpp

Index: Source/core/rendering/RenderListItem.cpp
diff --git a/Source/core/rendering/RenderListItem.cpp b/Source/core/rendering/RenderListItem.cpp
index 6c251c4263ad53e95a8804e375c1e8ec97d2d89d..df84cc1ae55f3626471955fd8edace91dcb812cd 100644
--- a/Source/core/rendering/RenderListItem.cpp
+++ b/Source/core/rendering/RenderListItem.cpp
@@ -63,8 +63,15 @@ void RenderListItem::styleDidChange(StyleDifference diff, const RenderStyle* old
         // The marker always inherits from the list item, regardless of where it might end
         // up (e.g., in some deeply nested line box). See CSS3 spec.
         newStyle->inheritFrom(style());
-        if (!m_marker)
+        if (m_marker) {
+            // Reuse the current margins otherwise resetting the margins to initial values

[mstensho (USE GERRIT), 2014/10/14 19:06:03]

I don't think RenderListItem should have to know that margin-left and
margin-right are the very properties that the marker renderer modifies, so
better deal with that piece of logic in RenderListMarker.

NB! It should not be margin-left and margin-right, but margin-start and
margin-end!

Maybe we could move all the style fiddling code here into a RenderListMarker
method, that:
0. Creates a RenderStyle
1. Inherits properties from the list-item
2. Overrides what it needs to override (for now, margin-start and margin-end).
3. Sets the new style on the marker.

[Xianzhu, 2014/10/14 19:33:59]

Good catch! Though set{margin-left, margin-right} == set{margin-start,
margin-end} but missed the vertical case.

Done.
Done.

+            // would trigger unnecessary layout.
+            newStyle->setMarginLeft(m_marker->style()->marginLeft());
+            newStyle->setMarginRight(m_marker->style()->marginRight());
+        } else {
             m_marker = RenderListMarker::createAnonymous(this);
+        }
+
         m_marker->setStyle(newStyle.release());
     } else if (m_marker) {
         m_marker->destroy();