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

This reverts commit 01bf114063e0e75adf5220a7911fa1a3c0201e01.
This relands commit 5b4cb28a9c971c9404870fc3fdfaa99e9ccc336a (as
originally landed).

The failures on the Windows bot are correlated, but not due to this
change. (The failures occurred before the change, and mysteriously went
green, for multiple cycles. It also failed after this change was
reverted again.)

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

Review URL: https://codereview.chromium.org/775483003