[CodeHealth] Remove obsolete comments

There will not be such specialized subclass with root-layer-scrolling.

Bug: None
Change-Id: I7deba376df09673e7da5c91b65a7c4386dabe3c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872925
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708097}

third_party/blink/renderer/core/frame/local_frame_view.h

@@ -904,8 +904,6 @@ class CORE_EXPORT LocalFrameView final
   bool frame_timing_requests_dirty_;
 
   // Exists only on root frame.
-  // TODO(bokan): crbug.com/484188. We should specialize LocalFrameView for the
-  // main frame.
   Member<RootFrameViewport> viewport_scrollable_area_;
 
   // Non-top-level frames a throttled until they are ready to run lifecycle