Downloader: Refactor to encapsulate single download operation
Downloader: Refactor to encapsulate single download operation

pozdnyakov/crosswalk-app-tools,hgl888/crosswalk-app-tools,davidmaxwaterman/crosswalk-app-tools,shawngao5/crosswalk-app-tools,shawngao5/crosswalk-app-tools,baleboy/crosswalk-app-tools,pozdnyakov/crosswalk-app-tools,crosswalk-project/crosswalk-app-tools,baleboy/crosswalk-app-tools,davidmaxwaterman/crosswalk-app-tools,baleboy/crosswalk-app-tools,crosswalk-project/crosswalk-app-tools,crosswalk-project/crosswalk-app-tools,hgl888/crosswalk-app-tools