Add test for reading v2 golden PrefixSet file.
Previous version changes used manually constructed files. Put in place
infrastructure to use a golden file, which should be more robust to future
version changes.
Since |shared_prefixes_| is randomly generated, PrefixSetRef
persists the version used in generating the golden file. Later
code can use ReadReferencePrefixes() as source material for
generating future golden files.
BUG=172527
Review URL: https://codereview.chromium.org/
237673011
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@264687 0039d316-1c4b-4281-b951-d872f2087c98