[SPv2] Respect transform-style in the Blink and cc property trees.

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

Index: third_party/WebKit/Source/platform/graphics/paint/README.md
diff --git a/third_party/WebKit/Source/platform/graphics/paint/README.md b/third_party/WebKit/Source/platform/graphics/paint/README.md
index 618a5fc976f1998ab16f2eb4db73174ec2852f7a..d7d79d183e7db65d6c3f5ab3c42633bed1f9a520 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/README.md
+++ b/third_party/WebKit/Source/platform/graphics/paint/README.md
@@ -53,6 +53,10 @@ Each transform node has:
   transform origin will rotate the plane about that point)
 * a pointer to the parent node, which defines the coordinate space relative to
   which the above should be interpreted
+* a pointer to a transform node which establishes a 3D rendering context (i.e. a
+  set of transform nodes whose content is 3D sorted)
+* an enum which specifies whether transforms of child transform nodes are
+  flattened into the plane of this transform node
 
 The parent node pointers link the transform nodes in a hierarchy (the *transform
 tree*), which defines how the transform for any painted content can be
@@ -66,9 +70,10 @@ may be created to establish a perspective matrix for descendant transforms in
 order to create the illusion of depth.
 ***
 
-*** aside
-TODO(jbroman): Explain flattening, etc., once it exists in the paint properties.
-***
+Note that, even though CSS does not permit it in the DOM, the transform tree can
+have nodes that preserve 3D but nonetheless participate in, and establish, no
+rendering context. For example, it is necessary to preserve the 3D character of
+the perspective transform, but this does not imply any 3D sorting.
 
 ### Clips