Generate getters/setters for border-image.

Currently, the border-image property is a 'storage_only' field, so
it has no generated public getters/setters. This patch changes it to an
'external' field so that their getters/setters can be generated as well.

Diff: https://gist.github.com/dc683ef590e3d4e88e1ed7f3a54ff16f/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2888323002
Cr-Commit-Position: refs/heads/master@{#473403}
3 files changed