Optimize collapsed border painting

third_party/WebKit/Source/core/style/CollapsedBorderValue.h

Index: third_party/WebKit/Source/core/style/CollapsedBorderValue.h
diff --git a/third_party/WebKit/Source/core/style/CollapsedBorderValue.h b/third_party/WebKit/Source/core/style/CollapsedBorderValue.h
index f780d7d0756ba66f6b30ba189ae823c51ea7fa41..9abf8db6b2469bd6c013edeb9cbcc86d41f80783 100644
--- a/third_party/WebKit/Source/core/style/CollapsedBorderValue.h
+++ b/third_party/WebKit/Source/core/style/CollapsedBorderValue.h
@@ -70,9 +70,21 @@ public:
         return color() == o.color() && isTransparent() == o.isTransparent() && isSameIgnoringColor(o);
     }
 
+    bool isVisible() const
+    {
+        return style() > BHIDDEN && !isTransparent() && exists();
+    }
+
     bool shouldPaint(const CollapsedBorderValue& tableCurrentBorderValue) const
     {
-        return style() > BHIDDEN && !isTransparent() && exists() && isSameIgnoringColor(tableCurrentBorderValue);
+        return isVisible() && isSameIgnoringColor(tableCurrentBorderValue);

[Julien - ping for review, 2016/01/08 14:18:15]

Shouldn't this be ASSERT(isVisible()) as we don't want to see invisible borders
when painting?

Overall we should add more performance ASSERTs to make sure we don't have a bug
adding those back down the road.

[Xianzhu, 2016/01/08 17:50:45]

A cell may have e.g. 2 visible borders and 2 invisible borders. In the case we
need to check shouldPaint() during painting also for the invisible borders, so
we can't assert here.

[Julien - ping for review, 2016/01/11 18:11:36]

OK!

+    }
+
+    bool isEquivalentForPainting(const CollapsedBorderValue& o) const
+    {
+        if (!isVisible() && !o.isVisible())
+            return true;
+        return equals(o);
     }
 
 private: