Generate enum/getters/setters/mappings for margin-*-collapse.

Currently, the margin-*-collapse 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 EMarginCollapse enum and
use the generated CSSValueID <-> EMarginCollapse mappings.

Diff: https://gist.github.com/nainar/885b33371967b71d827c7ae7dab14ea0/revisions

BUG=628043

Change-Id: I4d9f96a1fbe47d0bc1e1deeeea2c25b5a2217094
Reviewed-on: https://chromium-review.googlesource.com/535379
Commit-Queue: nainar <nainar@chromium.org>
Reviewed-by: Rune Lillesveen <rune@opera.com>
Cr-Commit-Position: refs/heads/master@{#479607}
4 files changed