net: Add tests for UploadDataStream::position().
net: Add tests for UploadDataStream::position().

This API wasn't tested at all.

BUG=72001
TEST=net_unittests


Review URL: http://codereview.chromium.org/9169091

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@119363 0039d316-1c4b-4281-b951-d872f2087c98

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