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

third_party/WebKit/Source/core/paint/README.md

 # `Source/core/paint`
 
 This directory contains implementation of painters of layout objects.
 
 ## Glossaries
 
 ### Stacked elements and stacking contexts
 
 This chapter is basically a clarification of [CSS 2.1 appendix E. Elaborate description
 of Stacking Contexts](http://www.w3.org/TR/CSS21/zindex.html).
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 In a subsequence paint, if the layer would generate exactly the same display items, we'll simply
 output a `CachedSubsequence` display item in place of the display items, and skip all paintings
 of the layer and its descendants in painting order. After painting, `PaintController` will
 replace `CacheSubsequence` with cached display items created in previous paintings.
 
 There are many conditions affecting
 *   whether we need to generate subsequence for a PaintLayer;
 *   whether we can use cached subsequence for a PaintLayer.
 See `shouldCreateSubsequence()` and `shouldRepaintSubsequence()` in `PaintLayerPainter.cpp` for
 the conditions.
+
+## Empty paint phase optimization
+
+During painting, we walk the layout tree multiple times for multiple paint phases. Sometimes
+a layer contain nothing needing a certain paint phase and we can skip tree walk for such
+empty phases. Now we have optimized `PaintPhaseDescendantBlockBackgroundsOnly`,
+`PaintPhaseDescendantOutlinesOnly` and `PaintPhaseFloat` for empty paint phases.
+
+During paint invalidation, we set the containing self-painting layer's `needsPaintPhaseXXX`
+flag if the object has something needing to be painted in the paint phase.
+
+During painting, we check the flag before painting a paint phase and skip the tree walk if
+the flag is not set.
+
+It's hard to clear a `needsPaintPhaseXXX` flag when a layer no longer needs the paint phase,
+so we never clear the flags.
+
+When layer structure changes, and we are not invalidate paint of the changed subtree,
+we need to manually update the `needsPaintPhaseXXX` flags. For example, if an object changes
+style and creates a self-painting-layer, we copy the flags from its containing self-painting
+layer to this layer, assuming that this layer needs all paint phases that its container
+self-painting layer needs.
+
+We could update the `needsPaintPhaseXXX` flags in a separate tree walk, but that would regress
+performance of the first paint. For slimming paint v2, we can update the flags during the
+pre-painting tree walk to simplify the logics.