[Downloads] Support resuming interrupted downloads from the downloads shelf.

The download shelf context menu will have a 'Resume' item for
interrupted downloads. This option is available if
'--enable-download-resumption' flag is enabled.

BUG=7648

Review URL: https://chromiumcodereview.appspot.com/14955004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@204710 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed