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

This reverts commit 5b4cb28a9c971c9404870fc3fdfaa99e9ccc336a.

The bot claims this causes ViewManagerTest.ReorderView (and maybe some
others) to crash. I find this hard to believe, but let's try reverting
anyway.

TBR=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/764333004