ClippingMediaSource provides a timeline where the period and window have the
same start/end positions, so when clipping a child timeline with a non-zero
offset between the window and period it is necessary to clear the offset then
apply the offset to the start/end positions used in the ClippingMediaPeriod.
Also add a message to clipping exceptions.
Also fix adjustment of seeks to the start of the clipped view.
Issue: #3888
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187292506
