Generate enum/getters/setters/mappings for text-orientation.

Currently, the text-orientation 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 ETextOrientation enum and use the generated
CSSValueID <-> ETextOrientation mappings.

Diff: https://gist.github.com/nainar/257662e77043a84cf3def269576e534a/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2913723002
Cr-Commit-Position: refs/heads/master@{#475880}
9 files changed