[WebSocket] Use the empty reason on fail.

WebSocket should not provide a fail message to scripts because
the fail message can contain information that help scripts to know
about user's local network circumstances.
Use the empty string for the closing reason.

BUG=275459
R=tyoshino

Review URL: https://codereview.chromium.org/38793003

git-svn-id: svn://svn.chromium.org/blink/trunk@160423 bbb929c8-8fbe-4397-9dbb-9b2b20218538