Block mixed PingLoader requests.

We should block HTTP requests in an HTTPS context whenever possible.
Pings are practically zero-impact to users, so blocking them via a mixed
content warning is very low risk.

Mozilla is making the same change: https://hg.mozilla.org/try/rev/5a9f4d5a8707

BUG=382798

Review URL: https://codereview.chromium.org/324013003

git-svn-id: svn://svn.chromium.org/blink/trunk@176681 bbb929c8-8fbe-4397-9dbb-9b2b20218538