delete file when doing Interrupt() if resume mode is invalid. The
partial file, if exists, can no longer be used to complete the download.

R=asanka@chromium.org
BUG=none
TEST=DownloadItemTest.UnresumableInterrupt

Review URL: https://codereview.chromium.org/183743019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255506 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255506