WebSocketHandshake doesn't need to be able to accept ExecutionContext

The only user is MainThreadWebSocketChannel and always Document is passed.

BUG=none

Review URL: https://codereview.chromium.org/98693007

git-svn-id: svn://svn.chromium.org/blink/trunk@164743 bbb929c8-8fbe-4397-9dbb-9b2b20218538