DevTools: improve styles sidebar ui, introduce tabbed pane in doecked to bottom / undocked mode.

R=pfeldman, apavlov
BUG=267759

Review URL: https://chromiumcodereview.appspot.com/21797002

git-svn-id: svn://svn.chromium.org/blink/trunk@155490 bbb929c8-8fbe-4397-9dbb-9b2b20218538