Revert "Add histograms to ResourceFetcher to measure memory cache hit rate."

This caused a PLT regression.

TBR=abarth@chromium.org
BUG=294382

Review URL: https://chromiumcodereview.appspot.com/23850007

git-svn-id: svn://svn.chromium.org/blink/trunk@158103 bbb929c8-8fbe-4397-9dbb-9b2b20218538