Update startup.js

Showing the download link in showDownload button view checks the following 
Modernizr tests to pass

Modernizr.download && Modernizr.bloburls && Modernizr.blobbuilder

In the Else condition , it expects the download iFrame should be present , else
throwing uncaught JS exception.

So the same check should be here also for creating the iFrame.
Merge pull request #51 from shidhincr/patch-1

Fixing the ShowDownload button JS exception in Chrome -  startup.js