downloads: Improve download rate estimation.

content/browser/download/base_file.cc

Index: content/browser/download/base_file.cc
diff --git a/content/browser/download/base_file.cc b/content/browser/download/base_file.cc
index 687c321c9c8d212d983b4492399ef8de41a0ad80..f31a0646761361b07206481f739ecf300029766e 100644
--- a/content/browser/download/base_file.cc
+++ b/content/browser/download/base_file.cc
@@ -137,6 +137,7 @@ DownloadInterruptReason BaseFile::AppendDataToFile(const char* data,
     len -= write_size;
     current_data += write_size;
     bytes_so_far_ += write_size;
+    rate_estimator_.AddBytes(write_size);
   }
 
   RecordDownloadWriteSize(data_len);
@@ -340,9 +341,7 @@ void BaseFile::ClearStream() {
 }
 
 int64 BaseFile::CurrentSpeedAtTime(base::TimeTicks current_time) const {
-  base::TimeDelta diff = current_time - start_tick_;
-  int64 diff_ms = diff.InMilliseconds();
-  return diff_ms == 0 ? 0 : bytes_so_far() * 1000 / diff_ms;
+  return rate_estimator_.GetBytesPerSecond();
 }
 
 DownloadInterruptReason BaseFile::LogNetError(