Generate enum/getters/setters/mappings for user-drag

Currently, the user-drag 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 EUserDrag enum and
use the generated CSSValueID <-> EUserDrag mappings.

Diff: https://gist.github.com/62b80577a855251fb6f37dcf096c29fb/revisions

Bug: 628043
Change-Id: I439066be43a5515ba117e1cee52c1f0f178d5b13
Reviewed-on: https://chromium-review.googlesource.com/536218
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#479977}
4 files changed