Let collapsed border drawings be cacheable

Source/platform/graphics/paint/DisplayItem.cpp

Index: Source/platform/graphics/paint/DisplayItem.cpp
diff --git a/Source/platform/graphics/paint/DisplayItem.cpp b/Source/platform/graphics/paint/DisplayItem.cpp
index 315147e50412147b37c3d6cc029a6b04b2323b49..2b7fa97b72950a583f5e7bf6c0ac460196dbe2b7 100644
--- a/Source/platform/graphics/paint/DisplayItem.cpp
+++ b/Source/platform/graphics/paint/DisplayItem.cpp
@@ -53,6 +53,19 @@ static WTF::String paintPhaseAsDebugString(int paintPhase)
 
 static WTF::String specialDrawingTypeAsDebugString(DisplayItem::Type type)
 {
+    if (type > DisplayItem::TableCollapsedBorderBase && type <= DisplayItem::TableCollapsedBorderLast) {
+        StringBuilder sb;
+        sb.append("TableCollapsedBorder");
+        if (type & DisplayItem::TableCollapsedBorderTop)
+            sb.append("Top");
+        if (type & DisplayItem::TableCollapsedBorderRight)
+            sb.append("Right");
+        if (type & DisplayItem::TableCollapsedBorderBottom)
+            sb.append("Bottom");
+        if (type & DisplayItem::TableCollapsedBorderLeft)
+            sb.append("Left");
+        return sb.toString();
+    }
     switch (type) {
         DEBUG_STRING_CASE(BoxDecorationBackground);
         DEBUG_STRING_CASE(Caret);