Use the WebRender clip chain API

The old clipping API has been removed from WebRender, so this switches
both legacy and new layout over to use the clip chain API in preparation
for the WebRender upgrade.

components/layout/display_list/items.rs

@@ -18,6 +18,7 @@ use gfx_traits::print_tree::PrintTree;
 use gfx_traits::{self, StackingContextId};
 use msg::constellation_msg::PipelineId;
 use net_traits::image::base::Image;
+use script_traits::compositor::ScrollTreeNodeId;
 use servo_geometry::MaxRect;
 use std::cmp::Ordering;
 use std::collections::HashMap;
@@ -31,6 +32,7 @@ use webrender_api::{
     FilterOp, GlyphInstance, GradientStop, ImageKey, MixBlendMode, PrimitiveFlags,
     ScrollSensitivity, Shadow, SpatialId, StickyOffsetBounds, TransformStyle,
 };
+use wr::ClipChainId;
 
 pub use style::dom::OpaqueNode;
 
@@ -369,6 +371,12 @@ pub struct ClipScrollNode {
 
     /// The type of this ClipScrollNode.
     pub node_type: ClipScrollNodeType,
+
+    /// The WebRender spatial id of this node assigned during WebRender conversion.
+    pub scroll_node_id: Option<ScrollTreeNodeId>,
+
+    /// The WebRender clip id of this node assigned during WebRender conversion.
+    pub clip_chain_id: Option<ClipChainId>,
 }
 
 impl ClipScrollNode {
@@ -378,6 +386,8 @@ impl ClipScrollNode {
             clip: ClippingRegion::from_rect(LayoutRect::zero()),
             content_rect: LayoutRect::zero(),
             node_type: ClipScrollNodeType::Placeholder,
+            scroll_node_id: None,
+            clip_chain_id: None,
         }
     }
 
@@ -400,6 +410,8 @@ impl ClipScrollNode {
             clip: ClippingRegion::from_rect(clip_rect),
             content_rect: LayoutRect::zero(), // content_rect isn't important for clips.
             node_type: ClipScrollNodeType::Clip(ClipType::Rounded(complex_region)),
+            scroll_node_id: None,
+            clip_chain_id: None,
         }
     }
 }