Fix DashDownloader failure for some multi segment representations

Issue: #3729

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190922866

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/offline/DashDownloader.java

@@ -116,7 +116,8 @@ public final class DashDownloader extends SegmentDownloader<DashManifest, Repres
         }
         }
       }
       }
 
 
-      int segmentCount = index.getSegmentCount(C.TIME_UNSET);
+      long periodDurationUs = manifest.getPeriodDurationUs(key.periodIndex);
+      int segmentCount = index.getSegmentCount(periodDurationUs);
       if (segmentCount == DashSegmentIndex.INDEX_UNBOUNDED) {
       if (segmentCount == DashSegmentIndex.INDEX_UNBOUNDED) {
         throw new DownloadException("Unbounded index for representation: " + key);
         throw new DownloadException("Unbounded index for representation: " + key);
       }
       }