Increase test coverage for -webkit-border-image.

Increase test coverage for -webkit-border-image.
https://bugs.webkit.org/show_bug.cgi?id=76473

Reviewed by Tony Chang.

Make sure we also set the width of the border in the shorthand. It will help
in the future to remove CSSBorderImageValue class without breaking this property.

* fast/css/getComputedStyle/getComputedStyle-border-image-expected.txt:
* fast/css/getComputedStyle/getComputedStyle-border-image.html:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105207 268f45cc-cd09-0410-ab3c-d52691b4dbfc