[css-flexbox] Add comment to IntrinsicContentBlockSize

As requested in https://crrev.com/c/1546002

R=dgrogan@chromium.org

Bug: 945214
Change-Id: I20f6d82971b3a1b4ab632f7ac30db56ff792c36f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546938
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646610}