DOMRectList: Remove comment about [LegacyArrayClass] support

[LegacyArrayClass] was removed from the spec in
https://github.com/w3c/fxtf-drafts/issues/280, and was dropped from WebIDL
altogether in https://github.com/heycam/webidl/pull/551.

Change-Id: I1d43d3af24dd61e3ec5e8a2423ce9e492c9f5e39
Reviewed-on: https://chromium-review.googlesource.com/1034060Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#554740}

third_party/blink/renderer/core/geometry/dom_rect_list.idl

@@ -8,7 +8,6 @@
 // DOMRectList only exists for compatibility with legacy web content.
 // When specifying a new API, use sequence<DOMRect> instead.
 
-// TODO: Spec also includes LegacyArrayClass that our IDL doesn't support yet.
 [ NoInterfaceObject ] interface DOMRectList {
     readonly attribute unsigned long length;
     [MeasureAs=ClientRectListItem] getter DOMRect? item(unsigned long index);