DevTools: [Styles] Do not reformat color keywords inside url(...) when editing property values

R=vsevik
BUG=278271

Review URL: https://chromiumcodereview.appspot.com/23458034

git-svn-id: svn://svn.chromium.org/blink/trunk@157961 bbb929c8-8fbe-4397-9dbb-9b2b20218538