Add a new UMA enum to record how many download uses parallel requests

content/browser/download/download_job.cc

Index: content/browser/download/download_job.cc
diff --git a/content/browser/download/download_job.cc b/content/browser/download/download_job.cc
index 1ed38b82fa8d83d02b5910ad2399e4b8780b35e6..b9dd3a97965c630193f814999e9c08699cf5f324 100644
--- a/content/browser/download/download_job.cc
+++ b/content/browser/download/download_job.cc
@@ -54,4 +54,8 @@ void DownloadJob::AddByteStream(std::unique_ptr<ByteStreamReader> stream_reader,
                  base::Passed(&stream_reader), offset, length));
 }
 
+bool DownloadJob::UsesParallelRequests() const {
+  return false;
+}
+
 }  // namespace content