Add histograms to ResourceFetcher to evaluate memory cache hit rate

This is a rework of https://chromiumcodereview.appspot.com/23734006 that was
reverted due to a regression in PLT. This patch introduces three histograms
that measures the count of dead resources used from cache, revalidated, or
loaded again. This will drive possible improvements that could be made to the
cache to  decrease memory usage on Android.

BUG=290193

Review URL: https://codereview.chromium.org/148323002

git-svn-id: svn://svn.chromium.org/blink/trunk@166121 bbb929c8-8fbe-4397-9dbb-9b2b20218538