Add kSupportedVersion constant to net::websockets

BUG=
TEST=compile

Review URL: https://codereview.chromium.org/84993002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237035 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237035