[LayoutNG] Stop casting LayoutNGFlexibleBox to LayoutFlexibleBox

LayoutNGFlexibleBox allowed itself to be casted to a LayoutFlexibleBox
but it wasn't one. This caused crashes, naturally.

I audited all the calls to IsFlexibleBox* and IsFlexItem*, updating to
NG where appropriate.

Bug: 845235, 909021
Change-Id: Ibb3ee32a5f7a78b5d46702ad7426597f23e39d2a
Reviewed-on: https://chromium-review.googlesource.com/c/1368690
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615341}
21 files changed