[SPDY] Do not advertise SPDY/2 by default

Add a switch to re-enable advertising SPDY/2.

Use SPDY/3 instead of SPDY/2 for cases where SPDY is forced on. Also use SPDY/3 instead of SPDY/2 in tests.

Add "deprecated" to all enum constants involving SPDY/2.

Remove unused --enable-npn switch.

BUG=303957
R=darin@chromium.org, rtenneti@chromium.org

Review URL: https://codereview.chromium.org/25977005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227229 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227229