Move ProxyServer to Source/platform/network/

(ignoring enum-related style nits in ProxyServer.h)

BUG=297477
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25646006

git-svn-id: svn://svn.chromium.org/blink/trunk@158928 bbb929c8-8fbe-4397-9dbb-9b2b20218538