DevTools: Migrate collectAllStyleSheets in css agent to the StyleEngine::activeStyleSheetsForInspector() method.

This fixes following problems:
 - shadow dom stylesheets were not reported in getAllStyleSheets protocol method.
 - shadow dom stylesheets were not send to the frontend after enabling css agent if frontend was restarted on the same page.

Review URL: https://codereview.chromium.org/119273003

git-svn-id: svn://svn.chromium.org/blink/trunk@164343 bbb929c8-8fbe-4397-9dbb-9b2b20218538