Currently EventMessage's presentationTimeMs is kept separately in
EventSampleStream. However, EventMessage's presentationTimeMs maybe used in
other places besides EventSampleStream, such as when handling `emsg' messages
targeting the player. This CL let EventMessage object to holds its
presentationTimeMs for such use cases.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176502938
