[Downloads] Add a CanResume() method to DownloadItem.

The method indicates whether a download can be resumed. I.e.  Calling
DownloadItem::Resume() is expected to do something. It will be used by
the UI when support for downloads resumption is added.

BUG=7648

Review URL: https://chromiumcodereview.appspot.com/14957002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@200719 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed