Add blob utils to help handle blob download through ajax

// execute operation
_executeOperation: function() {
  return this.$.operation.execute().then(function(response) {
    return this._download(response).then(function() {
      // success
      this.fire('document-updated');
    }.bind(this));
  }.bind(this)).catch(function(response) {
    // something went wrong
  }.bind(this));
},

// trigger download
_download: function(response) {
  var contentDisposition = response.headers.get('Content-Disposition');
  if (contentDisposition) {
    var filenameMatches = contentDisposition
        .match(/filename[^;=\n]*=([^;\n]*''([^;\n]*)|[^;\n]*)/).filter(function(match) { return !!match; });
    var filename = decodeURI(filenameMatches[filenameMatches.length - 1]);
    return response.blob().then(function(blob) {
      if (navigator.msSaveBlob) {
        // handle IE11 and Edge
        navigator.msSaveBlob(blob, filename);
      } else {
        var a = document.createElement('a');
        a.style = 'display: none';
        a.download = filename;
        a.href = URL.createObjectURL(blob);
        document.body.appendChild(a);
        a.click();
        document.body.removeChild(a);
        URL.revokeObjectURL(a.href);
      }
    }.bind(this));
  } else {
    return Promise.reject(new Error('missing Content-Disposition header'));
  }
}