update_engine: add logs and metrics for libcurl errors

curl_multi_perform may return code that is neither CURLM_OK or
CURLM_CALL_MULTI_PERFORM. When error returns we log them in
update_engine.log and send UMA metrics.

When update_engine does not get http response code from libcurl, we log
internal error code of the libcurl transfer for diagnosis.

Chrome CL to add the metrics enum is here: https://chromium-review.googlesource.com/c/chromium/src/+/1566150

BUG=chromium:927039
TEST=unittest

Change-Id: Ie8ce9dc0a6ce5ff6ffc2ff9425b652d125518558
Reviewed-on: https://chromium-review.googlesource.com/1562172
Commit-Ready: Sean Abraham <seanabraham@chromium.org>
Tested-by: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/omaha_request_action.h b/omaha_request_action.h
index 8e81af9..f006d69 100644
--- a/omaha_request_action.h
+++ b/omaha_request_action.h
@@ -125,6 +125,11 @@
 
   void TransferComplete(HttpFetcher* fetcher, bool successful) override;
 
+  void ReportUpdateCheckMetrics(
+      metrics::CheckResult result,
+      metrics::CheckReaction reaction,
+      metrics::DownloadErrorCode download_error_code) override;
+
   // Returns true if this is an Event request, false if it's an UpdateCheck.
   bool IsEvent() const { return event_.get() != nullptr; }