Move XMLHttpRequest related files to core/xmlhttprequest.

XMLHttpRequest is not related to other core/xml files (such as XPath,
XSLTProcessor and so on), so move files to a new directory.

Add core/xmlhttprequest/OWNERS.
This CL does not change the behavior.

BUG=426816

Review URL: https://codereview.chromium.org/666153003

git-svn-id: svn://svn.chromium.org/blink/trunk@184622 bbb929c8-8fbe-4397-9dbb-9b2b20218538