Bug 1261900: [MSE] P4. Only evict no longer used data from resource. r=gerald

dom/media/mediasource/TrackBuffersManager.cpp

@@ -1269,9 +1269,21 @@ TrackBuffersManager::CompleteCodedFrameProcessing()
 
   // 6. Remove the media segment bytes from the beginning of the input buffer.
   // Clear our demuxer from any already processed data.
-  // As we have handled a complete media segment, it is safe to evict all data
-  // from the resource.
-  mCurrentInputBuffer->EvictAll();
+  int64_t safeToEvict = std::min(
+    HasVideo()
+      ? mVideoTracks.mDemuxer->GetEvictionOffset(mVideoTracks.mLastParsedEndTime)
+      : INT64_MAX,
+    HasAudio()
+      ? mAudioTracks.mDemuxer->GetEvictionOffset(mAudioTracks.mLastParsedEndTime)
+      : INT64_MAX);
+  ErrorResult rv;
+  mCurrentInputBuffer->EvictBefore(safeToEvict, rv);
+  if (rv.Failed()) {
+    rv.SuppressException();
+    RejectProcessing(NS_ERROR_OUT_OF_MEMORY, __func__);
+    return;
+  }
+
   mInputDemuxer->NotifyDataRemoved();
   RecreateParser(true);