CSSImageValue::create should take a KURL instead of a String

To avoid bloating the size of CSSImageValue, m_url is still stored as a String.
If Blink ever gets an URLString class, this would be a good place to use it.

BUG=311682

Review URL: https://codereview.chromium.org/132023004

git-svn-id: svn://svn.chromium.org/blink/trunk@164886 bbb929c8-8fbe-4397-9dbb-9b2b20218538