Clamp seek-sentinel target time once, not every sentinel run
getDuration() asks CEHTML object, getRange().end uses getDuration()
Revert "getDuration() asks CEHTML object, getRange().end uses getDuration()"

This reverts commit f0494e020dfb00e13d50ff1185742ddfaee780b8.
Use getDuration() to detect end of media in complete sentinel, not getRange().end