Factor PaintArtifact out of DisplayItemList.

third_party/WebKit/Source/platform/graphics/paint/README.md

 # Platform paint code
 
 This directory contains the implementation of display lists and display
 list-based painting, except for code which requires knowledge of `core/`
 concepts, such as DOM elements and layout objects.
 
 This code is owned by the [paint team][paint-team-site].
 
 Slimming Paint v2 is currently being implemented. Unlike Slimming Paint v1, SPv2
 represents its paint artifact not as a flat display list, but as a list of
 drawings, and a list of paint chunks, stored together.
 
 This document explains the SPv2 world as it develops, not the SPv1 world it
 replaces.
 
 [paint-team-site]: https://www.chromium.org/developers/paint-team
 
 ## Paint artifact
 
-The SPv2 paint artifact consists of a list of display items (ideally mostly or
-all drawings), partitioned into *paint chunks* which define certain *paint
-properties* which affect how the content should be drawn or composited.
+The SPv2 [paint artifact](PaintArtifact.h) consists of a list of display items
+(ideally mostly or all drawings), partitioned into *paint chunks* which define
+certain *paint properties* which affect how the content should be drawn or
+composited.
 
 ## Paint properties
 
 Paint properties define characteristics of how a paint chunk should be drawn,
 such as the transform it should be drawn with. To enable efficient updates,
 a chunk's paint properties are described hierarchically. For instance, each
 chunk is associated with a transform node, whose matrix should be multiplied by
 its ancestor transform nodes in order to compute the final transformation matrix
 to the screen.
 
@@ skipping 73 matching lines @@
 When the new display items have been populated, clients call
 `commitNewDisplayItems`, which merges the previous artifact with the new data,
 producing a new paint artifact, where `CachedDisplayItem` objects have been
 replaced with the cached content from the previous artifact.
 
 At this point, the paint artifact is ready to be drawn or composited.
 
 *** aside
 TODO(jbroman): Explain invalidation.
 ***