Reland of Skip PaintLayer empty paint phases if it previously painted nothing

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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 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.
+so we never clear the flags. Instead, we use another set of flags (`previousPaintPhaseXXXWasEmpty`)
+to record if a painting of a phase actually produced nothing. We'll skip the next
+painting of the phase if the flag is set, regardless of the corresponding
+`needsPaintPhaseXXX` flag. We will clear the `previousPaintPhaseXXXWasEmpty` flags when
+we paint with different clipping, scroll offset or interest rect from the previous paint.
+
+We don't clear the `previousPaintPhaseXXXWasEmpty` flags when the layer is marked `needsRepaint`.
+Instead we clear the flag when the corresponding `needsPaintPhaseXXX` is set. This ensures that
+we won't clear `previousPaintPhaseXXXWasEmpty` flags when unrelated things changed which won't
+cause the paint phases to become non-empty.
 
 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.