Allow to have a transparent UA dependent background.

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 d125094d9487110782a9e6e574d0efdd50706ff8..e86f79c4598c9c7b5ba0db48390003874bdf9027 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -1285,8 +1285,10 @@ bool LayoutBox::getBackgroundPaintedExtent(LayoutRect& paintedExtent) const
 
 bool LayoutBox::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect) const
 {
+    // Instead of returning false unconditionnaly for the document, check if it
+    // is transparent.
     if (isDocumentElement() || backgroundStolenForBeingBody())
-        return false;
+        return !isTransparent();

[trchen, 2016/03/08 01:57:21]

This change is incorrect. The document element indeed never paints backgrounds,
and the body doesn't paint background if it is stolen as root background.

And I don't think it is related to the thing you want to achieve. If your
problem is things being considered opaque when they are not, why making more
things considered opaque helps?

 
     Color backgroundColor = resolveColor(CSSPropertyBackgroundColor);
     if (backgroundColor.hasAlpha())