DevTools: Make captureScreenshot protocol command hidden.

It has been published in v1.1 by mistake.

BUG=None
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/25536011

git-svn-id: svn://svn.chromium.org/blink/trunk@158748 bbb929c8-8fbe-4397-9dbb-9b2b20218538