NewWebSocketChannelImpl: Add ASSERTs to WebSocketHandleClient implementations

R=yhirano
BUG=none

Review URL: https://codereview.chromium.org/687953002

git-svn-id: svn://svn.chromium.org/blink/trunk@184631 bbb929c8-8fbe-4397-9dbb-9b2b20218538