AU: Expose the server's HTTP response code in HttpFetcher.

A step towards resolving 2394 -- we'll need to exponentially back
off on 500 and 503 as well as 502 if possible.

BUG=2394
TEST=unit tests, gmerged and made sure updates can happen

Change-Id: I7928e3af37f23ce1ba197315ec52ab0b2ed0dc4c

Review URL: http://codereview.chromium.org/3106038
diff --git a/libcurl_http_fetcher.cc b/libcurl_http_fetcher.cc
index a07a825..9989ba2 100644
--- a/libcurl_http_fetcher.cc
+++ b/libcurl_http_fetcher.cc
@@ -78,6 +78,7 @@
   bytes_downloaded_ = 0;
   resume_offset_ = 0;
   retry_count_ = 0;
+  http_response_code_ = 0;
   ResumeTransfer(url);
   CurlPerformOnce();
 }
@@ -105,6 +106,7 @@
     } else {
       LOG(ERROR) << "Unable to get http response code.";
     }
+    http_response_code_ = static_cast<int>(http_response_code);
 
     // we're done!
     CleanUp();