Generate enum/getters/setters/mappings for box-decoration-break.

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

Diff of generated files:
https://gist.github.com/3e209733463b455cb70fb4f1c95435c9/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2844083005
Cr-Commit-Position: refs/heads/master@{#471675}
5 files changed