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

This reverts commit 01bf114063e0e75adf5220a7911fa1a3c0201e01.
And unreverts commit 5b4cb28a9c971c9404870fc3fdfaa99e9ccc336a.

BUT: Keep the constants kTypeChannel/kTypeRawChannel the same as before 5b4cb28a9c.

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

Review URL: https://codereview.chromium.org/768253002