Generate enum/getters/setters/mappings for flex-direction

Currently, the flex-direction 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 EFlexDirection enum and
use the generated CSSValueID <-> EFlexDirection mappings.

Diff: https://gist.github.com/297d38d32b4b097536e73f7c1bd1b3d7/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2942393002
Cr-Original-Commit-Position: refs/heads/master@{#480688}
Committed: https://chromium.googlesource.com/chromium/src/+/d6df17438f4c6ae0a69e5650f63d0cd6c8d72a00
Review-Url: https://codereview.chromium.org/2942393002
Cr-Commit-Position: refs/heads/master@{#480993}
4 files changed