DevTools: Clean up InspectorStyleSheet, InspectorCSSAgent, and InspectorResourceAgent

R=vsevik

Review URL: https://chromiumcodereview.appspot.com/20091002

git-svn-id: svn://svn.chromium.org/blink/trunk@154991 bbb929c8-8fbe-4397-9dbb-9b2b20218538