Rename HttpProxy* classes to Proxy*. Move them into a net/proxy/ subdirectory.

      I'm making this change because proxy resolution is really not specific to the HTTP protocol.  We need to use the proxy service in our FTP implementation, for example.  I made a separate directory instead of just putting these in base, because I anticipate more files once we have our own PAC implementation.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@651 0039d316-1c4b-4281-b951-d872f2087c98