Add a setting to control the DateExtension and add an histogram

I want to run an experiment and meassure the impact on unload handler
runtime depending on whether the date extension is on or off

BUG=299746
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25265003

git-svn-id: svn://svn.chromium.org/blink/trunk@158617 bbb929c8-8fbe-4397-9dbb-9b2b20218538