Added backround-repeat-x and -y as keyword properties.

Now code like <element>.style['background-repeat-x'] = 'no-repeat' would work.

BUG=352140

Review URL: https://codereview.chromium.org/231153003

git-svn-id: svn://svn.chromium.org/blink/trunk@171568 bbb929c8-8fbe-4397-9dbb-9b2b20218538