Implement merging non-composited paint property nodes in the PACompositor.

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

 # `Source/core/paint`
 
 This directory contains implementation of painters of layout objects. It covers
 the following document lifecycle states:
 
 *   PaintInvalidation (`InPaintInvalidation` and `PaintInvalidationClean`)
 *   PrePaint (`InPrePaint` and `PrePaintClean`)
 *   Paint (`InPaint` and `PaintClean`)
 
 ## Glossaries
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 TODO(wangxianzhu): add details
 
 ## [`PrePaintTreeWalk`](PrePaintTreeWalk.h) (Slimming paint v2 only)
 
 During `InPrePaint` document lifecycle state, this class is called to walk the whole
 layout tree, beginning from the root FrameView, across frame boundaries. We do the
 following during the tree walk:
 
 *   Building paint property tree: creates paint property tree nodes for special
     things in the layout tree, including but not limit to: overflow clip, transform,
-    fixed-pos, animation, mask, filter, etc.
+    fixed-pos, animation, mask, filter, etc. Also sets direct compositing reasons to be
+    used later for compositing.
 
 *   Paint invalidation: Not implemented yet. TODO(wangxianzhu): add details after
     it's implemented.
 
 ## Paint result caching
 
 `PaintController` holds the previous painting result as a cache of display items.
 If some painter would generate results same as those of the previous painting,
 we'll skip the painting and reuse the display items from cache.
 
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 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.