Clear ChunkedDataPipeUploadDataStream buffer length.

When clearing ChunkedDataPipeUploadDataStream::buf_ also set
ChunkedDataPipeUploadDataStream::buf_len_ to zero as it signifies
the length of buf_ which is now gone.

Bug: None
Change-Id: I470b46fcb5cd0615e1841780e3c9744763c85265
Reviewed-on: https://chromium-review.googlesource.com/c/1333657Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607693}

services/network/chunked_data_pipe_upload_data_stream.cc

@@ -164,6 +164,7 @@ void ChunkedDataPipeUploadDataStream::OnSizeReceived(int32_t status,
     data_pipe_.reset();
     // Clear |buf_| as well, so it's only non-null while there's a pending read.
     buf_ = nullptr;
+    buf_len_ = 0;
 
     OnReadCompleted(status_);