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 61f1f4d90bb054a524b32e402f78111c7d1de5a0..5858a7fb8ed0ec4813a758da3b702a44506aa8eb 100644
--- a/content/browser/download/download_file_impl.h
+++ b/content/browser/download/download_file_impl.h
@@ -176,6 +176,15 @@ 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 target location is already written by another worker, the current

[xingliu, 2017/03/11 00:11:46]

nit%: We actually check this only before the first disk IO of the stream. Maybe
explicitly address this here.

This sgtm since we can avoid iterate the received slice vector on every buffer
read.

A half open request will stop when it hit a received slice when another CL
checked in to set the length of previous source streams, right?

[qinmin, 2017/03/14 23:24:50]

Done.
Yes, a half open request will stop when a later source stream starts writing the
first chunk. 
There are 2 scenarios we need to handle. Suppose there are stream A, B, and C,
all of them are half open, and are ordered by the offset.
When B starts writing, it will set the length limit on A, thus causing it to
stop early. that's addressed in another CL.
Now B continues to write and it surpasses the offset of C. If C starts writing
at this moment, it should immediately stop. That's addressed in this CL.

+  // 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);