Fix ComputedStyleBase.h.tmpl to generate accessors for fields on groups

This patch fixes the jinja templates to generate accessors for fields on
groups. It then uses the generated accessors in ComputedStyle.h

https://gist.github.com/nainar/f2816c46b990a49db3e2c3990cc9cdb1/revisions

BUG=710938

Review-Url: https://codereview.chromium.org/2855583002
Cr-Commit-Position: refs/heads/master@{#468912}
4 files changed