DevTools: NetworkPanel: fix - WS frames are not shown.

This is because injected stylesheet is removed by obsolete code.

BUG=

Review URL: https://codereview.chromium.org/660443002

git-svn-id: svn://svn.chromium.org/blink/trunk@183739 bbb929c8-8fbe-4397-9dbb-9b2b20218538