MessageBuffer: add CanFitEver()

The CanFitEver() method will be used to determine
whether the MessageBuffer can host an incoming
message, or if the incoming message requires
compression.

Bug: 31653003
Test: ./runtests.sh (on bullhead)
Change-Id: If09ae5bc6c7c80a71e490e945187bd6fc25529b1
3 files changed