Add --disable-xslt to disable XSLT

We'll use this flag to experiment with replacing our current XSLT
implementation with an extension.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/25318002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226326 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226326