function blobToBase64String(blob) {
        return blobToBinaryString(blob).then(btoa);
    }
Convert a <code>Blob</code> to a binary string.

Example:

```js
blobUtil.blobToBase64String(blob).then(function (base64String) {
  // success
}).catch(function (err) {
  // error
});
```

@param blob
@returns Promise that resolves with the binary string