CSP 1.1: Move workers to 'child-src' from 'script-src'.

This is a backwards incompatible change from CSP 1.0, so this patch
introduces a UseCounter as well to determine when we can safely switch
over. I expect usage of workers with CSP to be low, but verification is
essential.

BUG=336788

Review URL: https://codereview.chromium.org/149023007

git-svn-id: svn://svn.chromium.org/blink/trunk@166586 bbb929c8-8fbe-4397-9dbb-9b2b20218538