Fix WebCore.Scripts.ParsingBlocking.TimeBetweenLoadedAndCompiled UMA some more.

Resources which were already loaded and then revalidated were skewing the data:
load time was coming from the already loaded resource, compile time was the new
compilation time.

R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/562653005

git-svn-id: svn://svn.chromium.org/blink/trunk@181895 bbb929c8-8fbe-4397-9dbb-9b2b20218538