Mojo: DataPipe: Implement "may discard" mode for simple writes.

(Still need to implement for two-phase writes.)

R=darin@chromium.org

Review URL: https://codereview.chromium.org/98013005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243176 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243176