Use Content-Length if greater than current size

The server may temporarily report an incorrect size that is smaller
than the actual size. Content-Length is expected to always be
accurate, but its value does not correspond to the full file size
when resuming downloads. Use Content-Length only if it's bigger
than the currently known size.

Change-Id: I2cc06bfbd2349f21528047b4840f549fbe84964e