Make DownloadFileImpl handle multiple byte streams.

content/browser/download/mock_download_file.cc

Index: content/browser/download/mock_download_file.cc
diff --git a/content/browser/download/mock_download_file.cc b/content/browser/download/mock_download_file.cc
index 9d883c7a7ebc9a66ecfd4f1aa3e5760414143ec7..e8abcf2db07dccaad8cb6f4b555ff25ecea91b0f 100644
--- a/content/browser/download/mock_download_file.cc
+++ b/content/browser/download/mock_download_file.cc
@@ -27,4 +27,12 @@ MockDownloadFile::MockDownloadFile() {
 MockDownloadFile::~MockDownloadFile() {
 }
 
+void MockDownloadFile::AddByteStream(
+    std::unique_ptr<ByteStreamReader> stream_reader,
+    int64_t offset) {
+  // Gmock currently can't mock method that takes move-only parameters,
+  // delegate the EXPECT_CALL count to |DoAddByteStream|.
+  DoAddByteStream(stream_reader.get(), offset);
+}
+
 }  // namespace content