Add a helper method to truncate SourceStream's length

content/browser/download/download_file_impl.h

Index: content/browser/download/download_file_impl.h
diff --git a/content/browser/download/download_file_impl.h b/content/browser/download/download_file_impl.h
index c6a4ae20e7c7febea6ef04e71e3de130913ae9f9..c34a6102c40c3fc1f03bc3152343bc6c877ee289 100644
--- a/content/browser/download/download_file_impl.h
+++ b/content/browser/download/download_file_impl.h
@@ -106,6 +106,11 @@ class CONTENT_EXPORT DownloadFileImpl : public DownloadFile {
     // Called after successfully writing a buffer to disk.
     void OnWriteBytesToDisk(int64_t bytes_write);
 
+    // Given a data block that is already written, truncate the length of this
+    // object to avoid overwriting that block.
+    void TruncateLengthWithWrittenDataBlock(int64_t offset,
+                                            int64_t bytes_written);
+
     ByteStreamReader* stream_reader() const { return stream_reader_.get(); }
     int64_t offset() const { return offset_; }
     int64_t length() const { return length_; }