Add histograms to track the hit rate of the memory cache in Blink

https://chromiumcodereview.appspot.com/148323002/ introduced histograms that
track the hit rate of the MemoryCache on dead resources in Blink. This is the
change in histograms.xml.

BUG=290193

Review URL: https://codereview.chromium.org/138543007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248968 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248968