Switch to null root property tree nodes [spv2]

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 a2db6dbc2b1ebff2b45e4c571b304d65c5484747..b3f04b04c26c681d7f66ebbef27bde90302156aa 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/README.md
+++ b/third_party/WebKit/Source/platform/graphics/paint/README.md
@@ -31,9 +31,17 @@ 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.
 
-*** note
-Support for all paint properties has yet to be implemented in SPv2.
-***
+Paint properties are owned and stored by the object that creates them (see
+[ObjectPaintProperties](../../../core/paint/ObjectPaintProperties.h) and the
+paint properties on [FrameView](../../../core/frame/FrameView.h)). The complete
+set of property tree nodes used to paint a LayoutObject is cached on
+[ObjectPaintProperties](../../../core/paint/ObjectPaintProperties.h) in
+`LocalBorderBoxProperties`. There is an important distinction between the local
+border box properties and the properties owned by a layout object: a null
+property in [ObjectPaintProperties](../../../core/paint/ObjectPaintProperties.h)
+(e.g., `paintOffsetTranslation`) only implies that a specific LayoutObject
+doesn't create a property, but a null value in `LocalBorderBoxProperties`
+implies that the root property tree node should be used.
 
 *** aside
 TODO(jbroman): Explain the semantics of transforms, clips, scrolls and effects
@@ -42,7 +50,7 @@ as support for them is added to SPv2.
 
 ### Transforms
 
-Each paint chunk is associated with a [transform node](TransformPaintPropertyNode.h),
+Paint chunks can be associated with a [transform node](TransformPaintPropertyNode.h)
 which defines the coordinate space in which the content should be painted.
 
 Each transform node has:
@@ -80,7 +88,7 @@ imply any 3D sorting.
 
 ### Clips
 
-Each paint chunk is associated with a [clip node](ClipPaintPropertyNode.h),
+Paint chunks can be associated with a [clip node](ClipPaintPropertyNode.h)
 which defines the raster region that will be applied on the canvas when
 the chunk is rastered.
 
@@ -95,7 +103,7 @@ root space, intersects with the raster region defined by its parent clip node
 
 ### Scrolling
 
-Each paint chunk is associated with a [scroll node](ScrollPaintPropertyNode.h)
+Paint chunks can be associated with a [scroll node](ScrollPaintPropertyNode.h)
 which defines information about how a subtree scrolls so threads other than the
 main thread can perform scrolling. Scroll information includes:
 
@@ -111,7 +119,7 @@ scroll offset is stored as a 2d transform in the transform tree.
 
 ### Effects
 
-Each paint chunk is associated with an [effect node](EffectPaintPropertyNode.h),
+Paint chunks can be associated with an [effect node](EffectPaintPropertyNode.h)
 which defines the effect (opacity, transfer mode, filter, mask, etc.) that
 should be applied to the content before or as it is composited into the content
 below.