DevTools: [SSP] generate autocomplete color names from WebInspector.Color.Nicknames

This patch purges WebInspector.CSSMetadata._color and uses
WebInspector.Color.Nicknames instead for property autocopmletion.

BUG=432801
R=apavlov

Review URL: https://codereview.chromium.org/758923002

git-svn-id: svn://svn.chromium.org/blink/trunk@186024 bbb929c8-8fbe-4397-9dbb-9b2b20218538