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

Currently, the flex-wrap 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 EFlexWrap enum and
use the generated CSSValueID <-> EFlexWrap mappings.

Diff: https://gist.github.com/cf118a1b16d0d7f12b32dbf644be638a/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2944193002
Cr-Commit-Position: refs/heads/master@{#481078}
4 files changed