Bug 1046531 - [Dolphin][v1.4] Cannot open PDF file

* Add pdf support in MimeMapper.
* Update pdfjs manifest.webapp and view.js from master branch.
With the shared/js/download/download_helper.js in v1.4 these lines are needed to make pdf work. However in master branch the 'runAction' function will use DOMDownload.contentType if the type is not found in the MimeMapper. How about stick with the change in download_helper.js made in master branch?