Update PaintLayer::needsPaintPhaseXXX flags when add/remove layer on style change

third_party/WebKit/Source/core/paint/PaintLayer.h

Index: third_party/WebKit/Source/core/paint/PaintLayer.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.h b/third_party/WebKit/Source/core/paint/PaintLayer.h
index 2a28fab1e5e6925ffe7f4d3a4fbeb02656f76888..a47fc7a4a63ce38dd3a642cf8605f0c9a4b13746 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -660,6 +660,7 @@ public:
 
     // Used to skip PaintPhaseDescendantOutlinesOnly for layers that have never had descendant outlines.
     // Once it's set we never clear it because it's not easy to track if all outlines have been removed.
+    // For more details, see core/paint/REAME.md#Empty paint phase optimization.
     bool needsPaintPhaseDescendantOutlines() const { return m_needsPaintPhaseDescendantOutlines; }
     void setNeedsPaintPhaseDescendantOutlines() { ASSERT(isSelfPaintingLayer()); m_needsPaintPhaseDescendantOutlines = true; }
 
@@ -762,6 +763,13 @@ private:
         return *m_rareData;
     }
 
+    void mergeNeedsPaintPhaseFlagsFrom(const PaintLayer& layer)
+    {
+        m_needsPaintPhaseDescendantOutlines |= layer.m_needsPaintPhaseDescendantOutlines;
+        m_needsPaintPhaseFloat |= layer.m_needsPaintPhaseFloat;
+        m_needsPaintPhaseDescendantBlockBackgrounds |= layer.m_needsPaintPhaseDescendantBlockBackgrounds;
+    }
+
     unsigned m_layerType : 2; // PaintLayerType
 
     // Self-painting layer is an optimization where we avoid the heavy Layer painting