Instrument FreeType allocations.

This will allow reporting of memory usage by FreeType instances, which
are measured in the MiBs for LayoutWorker threads.

This change also makes FreeType allocations happen with jemalloc instead
of the system malloc, which is a good thing.

Finally, the change documents some dubiousness involving
FontContextHandle.

components/gfx/platform/macos/font_context.rs

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use util::mem::HeapSizeOf;
+
 #[derive(Clone)]
 pub struct FontContextHandle {
     ctx: ()
@@ -13,3 +15,9 @@ impl FontContextHandle {
         FontContextHandle { ctx: () }
     }
 }
+
+impl HeapSizeOf for FontContextHandle {
+    fn heap_size_of_children(&self) -> usize {
+        0
+    }
+}