Block mixed EventSource conections.

Similar to XMLHttpRequest and WebSockets, we should block mixed EventSource
connections by default.

BUG=389209

Review URL: https://codereview.chromium.org/351923004

git-svn-id: svn://svn.chromium.org/blink/trunk@177192 bbb929c8-8fbe-4397-9dbb-9b2b20218538