[DevTools] Fix debug toolbar drawer style.

BUG=389076
TBR=vsevik@chromium.org

Review URL: https://codereview.chromium.org/362243002

git-svn-id: svn://svn.chromium.org/blink/trunk@177381 bbb929c8-8fbe-4397-9dbb-9b2b20218538