Break http/tests/xmlhttprequest/origin-exact-matching into 48 (!!!) tests.

Synchronous XMLHttpRequest is bad. 48 synchronous XMLHttpRequests in one
layout test is very bad. This test was running up to ~91 seconds on Win,
which is no fun.

Breaking it up is a bit ugly, but whatever.

Review URL: https://codereview.chromium.org/166543003

git-svn-id: svn://svn.chromium.org/blink/trunk@167274 bbb929c8-8fbe-4397-9dbb-9b2b20218538