[Downloads] Update origin info after each response.

When a download is resumed, a new URL request is sent out. The response
received for this request may contain new ETag and Last-Modified
information which should be used for subsequent resumption attempts.
Otherwise if a resource changes (along with the corresponding ETag)
subsequent partial resumption attempts will all fail even if it should
have succeeded.

BUG=7648

Review URL: https://codereview.chromium.org/74523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240773 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240773