net: Move the logic reading upload elements to UploadData::Element.

This is yet another clean-up patch to simplify UploadDataStream
in preparation for making UploadDataStream::Read asynchronous.

Managing the offset and the file stream inside UploadData::Element
is cleaner anyway.

BUG=72001
TEST=net_unittests


Review URL: http://codereview.chromium.org/9452040

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@123677 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed