Mojo: DataPipe: Test "all or nothing" mode.

(Except for two-phase read/writes in "may discard" mode, which isn't
supported yet.)

R=davemoore@chromium.org

Review URL: https://codereview.chromium.org/126663003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243450 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243450