Bump the severity of blocked mixed content from Warning to Error.

No tests change based on this, as we don't currently emit the message's
level in WebTestProxyBase::didAddMessageToConsole. That's probably
worth changing, but it will be a pain in the ass to rebaseline all the
results.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/26465013

git-svn-id: svn://svn.chromium.org/blink/trunk@159329 bbb929c8-8fbe-4397-9dbb-9b2b20218538