Add documentation on cc/Blink nodes, ids, and indices.

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
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 consuming the `PaintArtifact` produced by the `PaintController`, and converting
 it into a form suitable for the compositor to consume.
 
 At present, `PaintArtifactCompositor` creates a cc layer tree, with one layer
 for each paint chunk. In the future, it is expected that we will use heuristics
 to combine paint chunks into a smaller number of layers.
 
 The owner of the `PaintArtifactCompositor` (e.g. `WebView`) can then attach its
 root layer to the overall layer hierarchy to be displayed to the user.
 
+In the future we would like to explore moving to a single shared property tree
+representation across both cc and
+Blink. See [Web Page Geometries](https://goo.gl/MwVIto) for more.
+
 ## Geometry routines
 
 The [`GeometryMapper`](GeometryMapper.h) is responsible for efficiently computing
 visual and transformed rects of display items in the coordinate space of ancestor
 [`PropertyTreeState`](PropertyTreeState.h)s.
 
 The transformed rect of a display item in an ancestor `PropertyTreeState` is
 that rect, multiplied by the transforms between the display item's
 `PropertyTreeState` and the ancestors, then flattened into 2D.
 
 The visual rect of a display item in an ancestor `PropertyTreeState` is the
 intersection of all of the intermediate clips (transformed in to the ancestor
 state), with the display item's transformed rect.