Fix CENC car MPD for IE & Firefox

The manifest didn't declare an XML namespace.  Firefox and IE are
very strict about this, whereas Chrome is not.

Change-Id: I96fd44bb2debb974d7587603732965982681eadd

assets/car_cenc-20120827-manifest.mpd

@@ -1,4 +1,4 @@
-<MPD xmlns="urn:mpeg:DASH:schema:MPD:2011" mediaPresentationDuration="PT0H3M1.63S" minBufferTime="PT1.5S" profiles="urn:mpeg:dash:profile:isoff-on-demand:2011"
+<MPD xmlns="urn:mpeg:DASH:schema:MPD:2011" xmlns:ytdrm="http://youtube.com/ytdrm" mediaPresentationDuration="PT0H3M1.63S" minBufferTime="PT1.5S" profiles="urn:mpeg:dash:profile:isoff-on-demand:2011"
 type="static">
   <BaseURL>//yt-dash-mse-test.commondatastorage.googleapis.com/media/car_cenc-20120827-manifest.mpd</BaseURL>
   <Period duration="PT0H3M1.63S" start="PT0S">

spec/mpd_spec.js

@@ -471,22 +471,6 @@ describe('mpd', function() {
     }
   });
 
-  it('parses namespaced elements', function() {
-    var source = [
-      '<MPD>',
-      '  <Period id="1" duration="PT0H3M1.63S" start="PT0S">',
-      '    <AdaptationSet id="1" lang="fr" contentType="audio">',
-      '      <ContentProtection schemeIdUri="com.bogus">',
-      '        <prefix:TagName keyid="0" key="1"/>',
-      '      </ContentProtection>',
-      '    </AdaptationSet>',
-      '  </Period>',
-      '</MPD>'].join('\n');
-
-    var mpd = shaka.dash.mpd.parseMpd(source, createFailover('').urls);
-    expect(mpd).toBeTruthy();
-  });
-
   it('defaults startNumber to 1 when missing', function() {
     var source = [
       '<MPD>',