TreatReturnedNullStringAs=undefined removed from WebSocket.idl
TreatReturnedNullStringAs=undefined removed from WebSocket.idl

DOMWebSocket::m_subprotocol and ::m_extensions are initialized as
empty string and are only updated in DOMWebSocket::didConnect(), where
the values ultimate come from WebSocketStream::GetSubProtocol() and
::GetExtensions() which are std::string in Chromium. These are both
documented as returning the empty string if no protocol/extensions
were negotiated, and std::string as no null state like WTF::String. In
other words, it's not possible for these to return null string, but
even if they did it would appropriate to convert that to empty strings
as opposed to undefined.

BUG=497982
R=philipj, haraken

Review URL: https://codereview.chromium.org/1175183004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@196948 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk