Avoid integer overflows in BlobURLRequestJob.

      BUG=169685


Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=179154

Review URL: https://chromiumcodereview.appspot.com/12047012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@179901 0039d316-1c4b-4281-b951-d872f2087c98