Add "received_bytes" getter to HttpSteamParser.

The final goal is to add "GetTransferSize" to url request,
so DevTools could reliably report it to user.
See: https://codereview.chromium.org/46223003/

In HttpStreamParser we calculate received_bytes as length
of headers plus total amount of data that represents body.

Data provided in read_buf is counted as well.

Intermediate values could be greater than final.
When transfer of body is finished, HttpStreamParser
places all unconsumed data back to read_buf; at this moment
"received_bytes" is corrected for corresponding amount.

BUG=111052

Review URL: https://codereview.chromium.org/84693007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239464 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239464