DataReference::vector() should be const.
https://bugs.webkit.org/show_bug.cgi?id=109928

Reviewed by Anders Carlsson.

* Platform/CoreIPC/DataReference.h:
(CoreIPC::DataReference::vector):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144909 268f45cc-cd09-0410-ab3c-d52691b4dbfc