net: Add some extra cache histograms.

Add one histogram to look at the prevalence of Vary:User_Agent and another
one to give the wall time for the cache's age.

BUG=none
TEST=none
R=gavinp@chromium.org

Review URL: https://codereview.chromium.org/25408002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228632 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228632