Revert 153563 - Remove DownloadFileManager in favor of direct ownership of DownloadFiles.

Revert 153563 - Remove DownloadFileManager in favor of direct ownership of DownloadFiles.

This CL appears to have caused SavePageBrowserTest.SaveViewSourceHTMLOnly to fail flakily.




This CL is equivalent to CLs http://codereview.chromium.org/10799005, 
http://codereview.chromium.org/10836293, and
https://chromiumcodereview.appspot.com/10823406, which were previous attempts 
to land the same functionality.  

TBR=jam@chromium.org

BUG=123998
BUG=144751
Review URL: https://chromiumcodereview.appspot.com/10867065

TBR=rdsmith@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=153593