Add static root property tree nodes [spv2]

third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h

Index: third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h b/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
index 00d2d2cc27bf56b86c6186e2e8a534d50d3fc5f0..a0ced2f271298c189aa9f50b6f1c925e5b7d69a7 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
@@ -53,6 +53,7 @@ public:
     }
 
     ScrollPaintPropertyNode* parent() const { return m_parent.get(); }
+    bool isRoot() const { return !m_parent; }
 
     // Transform that the scroll is relative to.
     const TransformPaintPropertyNode* scrollOffsetTranslation() const { return m_scrollOffsetTranslation.get(); }
@@ -79,6 +80,10 @@ public:
     {
         m_mainThreadScrollingReasons |= reasons;
     }
+    void clearMainThreadScrollingReasons()
+    {
+        m_mainThreadScrollingReasons = 0;
+    }
 
 private:
     ScrollPaintPropertyNode(