Add a primitive checksum to PrefixSet construction.

There's a bug where the PrefixSet doesn't seem to work in a small
fraction of cases.  One possibility is that memory is being corrupted
during construction.  Though this seems very unlikely, this should
make sure...

BUG=71832
TEST=Monitor histograms

Review URL: http://codereview.chromium.org/6706032

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@80256 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed