Retrieve per host storage usage from QuotaManager.

R=kinuko@chromium.org
BUG=none
TEST=QuotaManagerTest.GetUsage

Review URL: http://codereview.chromium.org/8079004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@103921 0039d316-1c4b-4281-b951-d872f2087c98
  GatherHostUsageTask(
      UsageTracker* tracker,
      QuotaClient* client,
      const std::string& host)
      : GatherUsageTaskBase(tracker, client),
        client_(client),
        host_(host),
        callback_factory_(ALLOW_THIS_IN_INITIALIZER_LIST(this)) {
    DCHECK(client_);
  }

  GatherHostUsageTask(
      UsageTracker* tracker,
      QuotaClient* client,
      const std::string& host)
      : GatherUsageTaskBase(tracker, client),
        client_(client),
        host_(host),
        callback_factory_(ALLOW_THIS_IN_INITIALIZER_LIST(this)) {
    DCHECK(client_);
  }
