DevTools: Bring back split view animation on show/hide sidebar button click.

R=dgozman@chromium.org, dgozman, pfeldman

Review URL: https://codereview.chromium.org/187833006

git-svn-id: svn://svn.chromium.org/blink/trunk@168623 bbb929c8-8fbe-4397-9dbb-9b2b20218538