Generate enum/getters/setters/mappings for text-overflow

Currently, the text-overflow 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 ETextOverflow enum and
use the generated CSSValueID <-> ETextOverflow mappings.

Diff of generated files:
https://gist.github.com/e1fab4f5465cda4efbc79c15ec5e2bfe/revisions

Bug: 628043
Change-Id: I43b2808181780dd4ad7c90200c1f84884b4629c3
Reviewed-on: https://chromium-review.googlesource.com/536217
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#480284}
4 files changed