Make background-image and border-*-style accept CSSKeywordValue('none') as an acceptable value

This patch makes background-image and border-*-style accept
CSSKeywordValue('none') as an acceptable value by editing the
CSSProperties.json5 file.

It also adds the LayoutTest that tests this change.

BUG=691455

Review-Url: https://codereview.chromium.org/2714663002
Cr-Commit-Position: refs/heads/master@{#453889}
7 files changed