This happens for example if
 the streams provide decode-only samples before the intended playback start position that need
 to be dropped.

 <p>After this method has returned a value other than <a href="../C.html#TIME_UNSET"><code>C.TIME_UNSET</code></a>, all <a href="SampleStream.html" title="interface in com.google.android.exoplayer2.source"><code>SampleStreams</code></a> provided by the period are guaranteed to start from a key frame.

 <p>This method is only called after the period has been prepared.</d