Add error code string to a DVLOG

I've noticed this log on the quic.rocks
server and I think including the string
in this log will facilitate debugging.

R=zhongyi@chromium.org

Change-Id: Iaf0744e3325310989ccac1dc216cf33162b1ed14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225481
Commit-Queue: David Schinazi <dschinazi@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Auto-Submit: David Schinazi <dschinazi@chromium.org>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773889}

net/base/net_errors_posix.cc

@@ -17,7 +17,8 @@ namespace net {
 
 Error MapSystemError(logging::SystemErrorCode os_error) {
   if (os_error != 0)
-    DVLOG(2) << "Error " << os_error;
+    DVLOG(2) << "Error " << os_error << ": "
+             << logging::SystemErrorCodeToString(os_error);
 
   // There are numerous posix error codes, but these are the ones we thus far
   // find interesting.