bug 957592

* Addressed peer review concerns by adding an enum for the DownloadState in Downloads.webidl
* Updated DownloadsAPI.js to reflect new enum for DownloadState
* Remove changes to test_interfaces.html originally committed in bug 938023.
* Make DOMDownload Object always visible to chrome windows.
* Remove duplicate pref("dom.mozDownloads.enabled", true); entry in b2g.js.
* If we're attempting to access DOM Downloads API objects from chrome we will always allow it. This is necessary based on how we had to implement the API itself.
* Made downloads api test more robust by fixing some silly bugs in serve_file.sjs (we were continuing to attempt to write to a closed response!)

r=bz