Move hardcoded extra fields to its own JSON5 file.

For extra fields in ComputedStyle, we have been hardcoding their
values in the generator script. However, this is ugly and not
sustainable. This patch moves them into its own JSON file.

This patch does not change generated code.

BUG=628043

Review-Url: https://codereview.chromium.org/2788353002
Cr-Commit-Position: refs/heads/master@{#466178}
3 files changed