[SPDY] Add switch and about:flags entry to enable SPDY/4a2

BUG=230124
R=darin@chromium.org, rch@chromium.org

Review URL: https://codereview.chromium.org/17764006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@209012 0039d316-1c4b-4281-b951-d872f2087c98
7 files changed