Generate getters/setters for border-spacing.

Currently, the border-spacing properties: border-horizontal-spacing and
border-vertical-spacing are 'storage_only' fields, so they don't have
generated public getters/setters. This patch changes them to 'primitive'
so that their getters/setters can be generated as well.

Diff of generated files:
https://gist.github.com/darrnshn/7f153ab5b642e1a9dd7fb9e0573adaa9/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2845233002
Cr-Commit-Position: refs/heads/master@{#472602}
3 files changed