Generate enum/getters/setters/mappings for box-sizing.

Currently, the box-sizing property is a 'storage_only' field, so
it has no generated public getters/setters. This patch changes it to a
'keyword' field so that their getters/setters can be generated as well.
We also generate the EBoxSizing enum and use the generated CSSValueID
<-> EBoxSizing mappings.

Diff of generated files:
https://gist.github.com/darrnshn/cb6f24173e65563f8a2c755a4db10836/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2853433003
Cr-Commit-Position: refs/heads/master@{#471665}
5 files changed