Add a performance test for sending a batch of XMLHttpRequests

R=tonyg@chromium.org
BUG=348655

Review URL: https://codereview.chromium.org/177983010

git-svn-id: svn://svn.chromium.org/blink/trunk@168337 bbb929c8-8fbe-4397-9dbb-9b2b20218538