Move conent::Referrer serialization to use IPC macros.

This is just a struct, so there is no reason to have hand-written
serialization.  I thought I had landed this some time ago, but
apparently not. Doing this will allow more messages to be generated
in the IPC fuzzer.

Review URL: https://codereview.chromium.org/132023008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247019 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247019