The download button in pdf.js doesn't work in iOS Chrome.
- It appears to be an issue with URLs from URL.createObjectURL.
The URL is correct, but iOS Chrome won't even load the URL
when `a.click()` is called in `download_manager.js`. Even
if you manually visit the URL, you get a blank page.
- Fix this by detecting iOS Chrome and disabling createObjectURL.
The `download_manager.js` `download` method wasn't checking
`PDFJS.disableCreateObjectURL`, so check it there, too.
- Move the navigator.msSaveBlob check earlier, so that
this doesn't change IE10 / IE11 behavior.
- Remove the !URL check since pdf.js has a URL polyfill
now.
4fc64ceb76