Allow URLRequest::AppendChunkToUpload to take 0-byte final chunks.

Also add HttpStreamParser unit tests for 0 byte final chunks.

BUG=405678

Review URL: https://codereview.chromium.org/921453003

Cr-Commit-Position: refs/heads/master@{#317051}

net/url_request/url_request.cc

@@ -209,7 +209,6 @@ void URLRequest::AppendChunkToUpload(const char* bytes,
                                      bool is_last_chunk) {
   DCHECK(upload_data_stream_);
   DCHECK(upload_data_stream_->is_chunked());
-  DCHECK_GT(bytes_len, 0);
   upload_chunked_data_stream_->AppendData(bytes, bytes_len, is_last_chunk);
 }
 

net/url_request/url_request.h

@@ -303,10 +303,14 @@ class NET_EXPORT URLRequest : NON_EXPORTED_BASE(public base::NonThreadSafe),
   void EnableChunkedUpload();
 
   // Appends the given bytes to the request's upload data to be sent
-  // immediately via chunked transfer encoding. When all data has been sent,
-  // call MarkEndOfChunks() to indicate the end of upload data.
+  // immediately via chunked transfer encoding. When all data has been added,
+  // set |is_last_chunk| to true to indicate the end of upload data.  All chunks
+  // but the last must have |bytes_len| > 0.
   //
   // This method may be called only after calling EnableChunkedUpload().
+  //
+  // Despite the name of this method, over-the-wire chunk boundaries will most
+  // likely not match the "chunks" appended with this function.
   void AppendChunkToUpload(const char* bytes,
                            int bytes_len,
                            bool is_last_chunk);