Add the ability to print the fastest N% of tests to print-layout-test-times.

R=ojan@chromium.org, abarth@chromium.org, peter@chromium.org
BUG=289883

Review URL: https://chromiumcodereview.appspot.com/23496066

git-svn-id: svn://svn.chromium.org/blink/trunk@157990 bbb929c8-8fbe-4397-9dbb-9b2b20218538