[Downloads] Some UMA for downloads resumption.

* Count number of requests initiated due to resumption.

* Count downloads that have a strong validator and supports range
  requests, since those are candidates for partial resumption.

* Count downloads that are interrupted after the associated WebContents
  is destroyed.

BUG=7648

Review URL: https://chromiumcodereview.appspot.com/20609004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@214461 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed