--- a/dom/media/webm/WebMDemuxer.cpp
+++ b/dom/media/webm/WebMDemuxer.cpp
@@ -1026,10 +1026,21 @@ WebMTrackDemuxer::GetBuffered()
 }
 
 void
 WebMTrackDemuxer::BreakCycles()
 {
   mParent = nullptr;
 }
 
+int64_t
+WebMTrackDemuxer::GetEvictionOffset(const media::TimeUnit& aTime)
+{
+  int64_t offset;
+  if (!mParent->GetOffsetForTime(aTime.ToNanoseconds(), &offset)) {
+    return 0;
+  }
+
+  return offset;
+}
+
 #undef WEBM_DEBUG
 } // namespace mozilla

--- a/dom/media/webm/WebMDemuxer.h
+++ b/dom/media/webm/WebMDemuxer.h
@@ -225,16 +225,18 @@ public:
   void Reset() override;
 
   nsresult GetNextRandomAccessPoint(media::TimeUnit* aTime) override;
 
   RefPtr<SkipAccessPointPromise> SkipToNextRandomAccessPoint(media::TimeUnit aTimeThreshold) override;
 
   media::TimeIntervals GetBuffered() override;
 
+  int64_t GetEvictionOffset(const media::TimeUnit& aTime) override;
+
   void BreakCycles() override;
 
 private:
   friend class WebMDemuxer;
   ~WebMTrackDemuxer();
   void UpdateSamples(nsTArray<RefPtr<MediaRawData>>& aSamples);
   void SetNextKeyFrameTime();
   RefPtr<MediaRawData> NextSample ();