Possible content instream positions.

CONTENT_INSTREAM_POSITION_UNSPECIFIED Content instream position is not specified in this version. This enum is a place holder for a default value and does not represent a real in stream ad position.
CONTENT_INSTREAM_POSITION_PRE_ROLL Ads that play before streaming content.
CONTENT_INSTREAM_POSITION_MID_ROLL Ads that play between the beginning and end of streaming content.
CONTENT_INSTREAM_POSITION_POST_ROLL Ads that play at the end of streaming content.
CONTENT_INSTREAM_POSITION_UNKNOWN Ads instream position is unknown.