Measure LocalLayoutContexts in LayoutTask and LayoutWorkers.

The FreeType instances in layout workers are reasonably large and worth
measuring. The one in LayoutTask is smaller but it's easy to measure at the
same time. Sample output:
```
|    8.33 MiB -- pages
|       8.33 MiB -- url(file:///home/njn/moz/servo/../servo-static-suite/wikipedia/Guardians%20of%20the%20Galaxy%20(film)%20-%20Wikipedia,%20the%20free%20encyclopedia.html)
|          1.32 MiB -- layout-worker-0-local-context
|          1.31 MiB -- layout-worker-1-local-context
|          1.24 MiB -- layout-worker-3-local-context
|          1.17 MiB -- layout-worker-4-local-context
|          1.08 MiB -- layout-worker-2-local-context
|          1.06 MiB -- layout-worker-5-local-context
|          0.78 MiB -- paint-task
|             0.78 MiB -- buffer-map
|          0.38 MiB -- layout-task
|             0.30 MiB -- display-list
|             0.07 MiB -- local-context
```
This required adding a mechanism to WorkQueue to measure worker TLSes.

components/layout/context.rs

@@ -29,6 +29,7 @@ use std::sync::mpsc::{channel, Sender};
 use style::selector_matching::Stylist;
 use url::Url;
 use util::geometry::Au;
+use util::mem::HeapSizeOf;
 use util::opts;
 
 struct LocalLayoutContext {
@@ -37,8 +38,21 @@ struct LocalLayoutContext {
     style_sharing_candidate_cache: RefCell<StyleSharingCandidateCache>,
 }
 
+impl HeapSizeOf for LocalLayoutContext {
+    // FIXME(njn): measure other fields eventually.
+    fn heap_size_of_children(&self) -> usize {
+        self.font_context.heap_size_of_children()
+    }
+}
+
 thread_local!(static LOCAL_CONTEXT_KEY: RefCell<Option<Rc<LocalLayoutContext>>> = RefCell::new(None));
 
+pub fn heap_size_of_local_context() -> usize {
+    LOCAL_CONTEXT_KEY.with(|r| {
+        r.borrow().clone().map_or(0, |context| context.heap_size_of_children())
+    })
+}
+
 fn create_or_get_local_context(shared_layout_context: &SharedLayoutContext)
                                -> Rc<LocalLayoutContext> {
     LOCAL_CONTEXT_KEY.with(|r| {

components/layout/layout_task.rs

@@ -9,7 +9,7 @@
 
 use animation;
 use construct::ConstructionResult;
-use context::{SharedLayoutContext, SharedLayoutContextWrapper};
+use context::{SharedLayoutContext, SharedLayoutContextWrapper, heap_size_of_local_context};
 use css::node_style::StyledNode;
 use data::{LayoutDataAccess, LayoutDataWrapper};
 use display_list_builder::ToGfxColor;
@@ -575,10 +575,28 @@ impl LayoutTask {
         let rw_data = self.lock_rw_data(possibly_locked_rw_data);
         let stacking_context = rw_data.stacking_context.as_ref();
         reports.push(Report {
-            path: path!["pages", format!("url({})", self.url), "display-list"],
+            path: path!["pages", format!("url({})", self.url), "layout-task", "display-list"],
             size: stacking_context.map_or(0, |sc| sc.heap_size_of_children()),
         });
 
+        // The LayoutTask has a context in TLS...
+        reports.push(Report {
+            path: path!["pages", format!("url({})", self.url), "layout-task", "local-context"],
+            size: heap_size_of_local_context(),
+        });
+
+        // ... as do each of the LayoutWorkers, if present.
+        if let Some(ref traversal) = rw_data.parallel_traversal {
+            let sizes = traversal.heap_size_of_tls(heap_size_of_local_context);
+            for (i, size) in sizes.iter().enumerate() {
+                reports.push(Report {
+                    path: path!["pages", format!("url({})", self.url),
+                                format!("layout-worker-{}-local-context", i)],
+                    size: *size
+                });
+            }
+        }
+
         reports_chan.send(reports);
     }