Eliminate ScrollRootId

Just use WebRender's ClipId directly. This will allow us to create and
use ReferenceFrames in the future, if we need to do that. It will also
make it easier to have Servo responsible for creating the root
scrolling area, which will allow removing some old hacks in the future.

components/layout/flow.rs

@@ -35,10 +35,11 @@ use floats::{Floats, SpeculatedFloatPlacement};
 use flow_list::{FlowList, MutFlowListIterator};
 use flow_ref::{FlowRef, WeakFlowRef};
 use fragment::{CoordinateSystem, Fragment, FragmentBorderBoxIterator, Overflow};
-use gfx_traits::{ScrollRootId, StackingContextId};
+use gfx_traits::StackingContextId;
 use gfx_traits::print_tree::PrintTree;
 use inline::InlineFlow;
 use model::{CollapsibleMargins, IntrinsicISizes, MarginCollapseInfo};
+use msg::constellation_msg::PipelineId;
 use multicol::MulticolFlow;
 use parallel::FlowParallelInfo;
 use serde::ser::{Serialize, SerializeStruct, Serializer};
@@ -62,6 +63,7 @@ use table_colgroup::TableColGroupFlow;
 use table_row::TableRowFlow;
 use table_rowgroup::TableRowGroupFlow;
 use table_wrapper::TableWrapperFlow;
+use webrender_traits::ClipId;
 
 /// Virtual methods that make up a float context.
 ///
@@ -429,8 +431,14 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
     /// children of this flow.
     fn print_extra_flow_children(&self, _: &mut PrintTree) { }
 
-    fn scroll_root_id(&self) -> ScrollRootId {
-        base(self).scroll_root_id
+    fn scroll_root_id(&self, pipeline_id: PipelineId) -> ClipId {
+        match base(self).scroll_root_id {
+            Some(id) => id,
+            None => {
+                warn!("Tried to access scroll root id on Flow before assignment");
+                pipeline_id.root_scroll_node()
+            }
+        }
     }
 }
 
@@ -961,7 +969,7 @@ pub struct BaseFlow {
     /// list construction.
     pub stacking_context_id: StackingContextId,
 
-    pub scroll_root_id: ScrollRootId,
+    pub scroll_root_id: Option<ClipId>,
 }
 
 impl fmt::Debug for BaseFlow {
@@ -1104,8 +1112,8 @@ impl BaseFlow {
             flags: flags,
             writing_mode: writing_mode,
             thread_id: 0,
-            stacking_context_id: StackingContextId::new(0),
-            scroll_root_id: ScrollRootId::root(),
+            stacking_context_id: StackingContextId::root(),
+            scroll_root_id: None,
         }
     }