Auto merge of #16531 - mrobinson:clip-id, r=glennw

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.

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components/script/dom/document.rs

@@ -93,7 +93,6 @@ use dom_struct::dom_struct;
 use encoding::EncodingRef;
 use encoding::all::UTF_8;
 use euclid::point::Point2D;
-use gfx_traits::ScrollRootId;
 use html5ever_atoms::{LocalName, QualName};
 use hyper::header::{Header, SetCookie};
 use hyper_serde::Serde;
@@ -143,6 +142,7 @@ use time;
 use timers::OneshotTimerCallback;
 use url::Host;
 use url::percent_encoding::percent_decode;
+use webrender_traits::ClipId;
 
 /// The number of times we are allowed to see spurious `requestAnimationFrame()` calls before
 /// falling back to fake ones.
@@ -699,9 +699,11 @@ impl Document {
 
         if let Some((x, y)) = point {
             // Step 3
+            let global_scope = self.window.upcast::<GlobalScope>();
+            let webrender_pipeline_id = global_scope.pipeline_id().to_webrender();
             self.window.perform_a_scroll(x,
                                          y,
-                                         ScrollRootId::root(),
+                                         ClipId::root_scroll_node(webrender_pipeline_id),
                                          ScrollBehavior::Instant,
                                          target.r());
         }