Also remove the comment on LayoutDescriptor as we want to pretenure
those.

Bug: chromium:738865
Change-Id: I5bdf66d383b481edc3250623e155b97d1081a7cc
Reviewed-on: https://chromium-review.googlesource.com/590235Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46970}