DevTools: make dark gray gutter when emulating device metrics.

BUG=305633
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/52603002

git-svn-id: svn://svn.chromium.org/blink/trunk@160949 bbb929c8-8fbe-4397-9dbb-9b2b20218538