DevTools: Implement drawer views as tabs in a sllding tabbed pane

BUG=305510
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/26734007

git-svn-id: svn://svn.chromium.org/blink/trunk@159761 bbb929c8-8fbe-4397-9dbb-9b2b20218538