Set XHR response MimeType to 'text/xml'.
Set XHR response MimeType to 'text/xml'.

- If the MimeType of the DASH presentation returned from the
  server is not 'text/xml' then responseXML would be NULL.

Change-Id: Ic863ea51aa6a48a993ccc0c37ecb8c26ec114b3f

gshORTON/webm.webm-dash-javascript,Suvarna1488/webm.webm-dash-javascript,matanbs/webm.webm-dash-javascript,reimaginemedia/webm.webm-dash-javascript,kleopatra999/webm.webm-dash-javascript,iniwf/webm.webm-dash-javascript,abwiz0086/webm.webm-dash-javascript,ericmckean/webm.webm-dash-javascript,kleopatra999/webm.webm-dash-javascript,kalli123/webm.webm-dash-javascript,ericmckean/webm.webm-dash-javascript,Suvarna1488/webm.webm-dash-javascript,Acidburn0zzz/webm.webm-dash-javascript,kalli123/webm.webm-dash-javascript,gshORTON/webm.webm-dash-javascript,Acidburn0zzz/webm.webm-dash-javascript,reimaginemedia/webm.webm-dash-javascript,altogother/webm.webm-dash-javascript,matanbs/webm.webm-dash-javascript,iniwf/webm.webm-dash-javascript,altogother/webm.webm-dash-javascript,abwiz0086/webm.webm-dash-javascript