[css-typed-om] Ensure StylePropertyMapReadonly IDL matches spec.

Spec added a new .size member, which we implement in this patch.

Bug: 812915
Change-Id: Ib48fc7b8053c41e4c79dc9cc280bc18376e5fbe0
Reviewed-on: https://chromium-review.googlesource.com/930101
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538304}
15 files changed