Mojo: More data pipe work -- consumer-side dispatcher.

Define/implement DataPipeConsumerDispatcher, and the parts of the
DataPipe base class that it interacts with.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/101623013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240809 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240809