Mojo: Some DataPipe cleanup/simplification.

Merge some state (as indicated by the now-deleted FIXME):
- two_phase_max_num_bytes_written_ with producer_in_two_phase_write_
- two_phase_max_num_bytes_read_ with consumer_in_two_phase_read_

R=davemoore@chromium.org, davemoore

Review URL: https://codereview.chromium.org/114763005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242779 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242779