private void onDataSourceCancelled() {
    setFailure(new CancellationException());
  }
Data source that wraps number of other data sources and waits until all of them are finished.
After that each call to getResult() returns list of final results of wrapped data sources. Caller
of getResult() is responsible for closing all each of the results separately.

<p>This data source does not propagate intermediate results.

@param <T>
imagepipeline/src/main/java/com/facebook/imagepipeline/datasource/ListDataSource.java