Move the logic to determine how much data can be written to another function

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 d1ad10f571dd340482ff67ff951f71b6e31a29b8..a0fc81585cf5d5a4b832590e1a57edf1b2fb2615 100644
--- a/content/browser/download/download_file_impl.h
+++ b/content/browser/download/download_file_impl.h
@@ -183,6 +183,16 @@ class CONTENT_EXPORT DownloadFileImpl : public DownloadFile {
   // Called before the data is written to disk.
   void WillWriteToDisk(size_t data_len);
 
+  // For a given SourceStream object and the bytes it has read, determine the
+  // actual number of bytes it can write to the disk. For parallel downloading,
+  // if the first disk IO writes to a location that is already written by
+  // another stream, the current stream should stop writing. Returns true if the
+  // stream can write no more data and should be finished, returns false
+  // otherwise.
+  bool CalculateBytesToWrite(SourceStream* source_stream,
+                             size_t bytes_read,
+                             size_t* bytes_to_write);
+
   // Called when there's some activity on the byte stream that needs to be
   // handled.
   void StreamActive(SourceStream* source_stream);