Revert "Revert "Revert "Remove MessageInTransit::kTypeMessagePipe; rename kTypeMessagePipeEndpoint to kTypeEndpoint; etc."""

This reverts commit c3737d666bc66c8fb2f4eb165d670b598d02e6b9.

Sigh.

TBR=yzshen@chromium.org,viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/775473002