Add the responseType parameter to InspectorTest.makeXHR

We are planning to refine XHR with responseType='blob'.
To test that the Inspector works with the new implementation,
we add the ability of specifying responseType to InspectorTest.makeXHR.

BUG=269055

Review URL: https://chromiumcodereview.appspot.com/23464028

git-svn-id: svn://svn.chromium.org/blink/trunk@157357 bbb929c8-8fbe-4397-9dbb-9b2b20218538