Cleaned up make_computed_style_base.py to be better self-documenting

Cleaned up make_computed_style_base.py to have more explicit
dependencies between fields, explain fields and field families, and
generally be better self-documented.

BUG=628043

Review-Url: https://codereview.chromium.org/2549373002
Cr-Commit-Position: refs/heads/master@{#437107}
1 file changed