Accessing Trac from build.webkit.org/dashboard should enable XMLHttpRequest.withCrede...

Accessing Trac from build.webkit.org/dashboard should enable XMLHttpRequest.withCredentials
https://bugs.webkit.org/show_bug.cgi?id=126391

Reviewed by Dan Bernstein.

Make it conditional, as trac.webkit.org doesn't work when withCredentials is enabled.
One can request credentials use via Initialization.js.

* BuildSlaveSupport/build.webkit.org-config/public_html/dashboard/Scripts/Trac.js:
(Trac):
(Trac.prototype.update):
* BuildSlaveSupport/build.webkit.org-config/public_html/dashboard/Scripts/Utilities.js:
(loadXML):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@161216 268f45cc-cd09-0410-ab3c-d52691b4dbfc