Prevent sandboxed iframes from bypassing WebSocket mixed content checks.

This patch ensures that we check both the current context and the
top-level browsing context when evaluating whether or not an insecure
websocket connection should be blocked as mixed content.

BUG=389040

Review URL: https://codereview.chromium.org/351163002

git-svn-id: svn://svn.chromium.org/blink/trunk@177070 bbb929c8-8fbe-4397-9dbb-9b2b20218538