Cleanup histograms tracking prefix-set issues in safe-browsing.

Remove code for histograms which aren't yielding additional useful
info.  Cleanup naming slightly.

BUG=71832
TEST=none


Review URL: https://chromiumcodereview.appspot.com/10802098

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148301 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed