Add a WebSettings item to turn off mixed content check for WebSocket

https://codereview.chromium.org/222153002/ will ban use of insecure
WebSockets from HTTPS origin. To give time for app developers to fix their
app and roll-out fixed version widely to get ready for this change, we'll
provide a WebSettings item to turn off the security check.

To limit this exception to WebSockets, I added a new item that controls
only WebSockets.

After this CL is landed, I'll land
https://codereview.chromium.org/248863003/ and then
https://codereview.chromium.org/222153002/

BUG=85271

Review URL: https://codereview.chromium.org/246893014

git-svn-id: svn://svn.chromium.org/blink/trunk@172454 bbb929c8-8fbe-4397-9dbb-9b2b20218538