Use generated getters/setters in ComputedStyle instead of directly accessing fields

Use generated getters/setters from
https://codereview.chromium.org/2786883002 in ComputedStyle instead of directly
accessing fields.

It also marks all generated fields as private to ensure no direct usage.

BUG=628043

Review-Url: https://codereview.chromium.org/2830573002
Cr-Commit-Position: refs/heads/master@{#465973}
3 files changed