Add detailed histograms for DownloadProtectionService URLFetcher errors.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index d224471e86ff4a09869d769d38fd679c6bfb8316..f774fd4860c23fb36fb67b67157026c1f188215a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12417,6 +12417,19 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="SBClientDownload.DownloadRequestNetError" enum="NetErrorCodes">
+  <summary>
+    The net error code for all CheckClientDownloadRequest URLFetchers.

[Ilya Sherman, 2013/07/25 03:30:52]

nit: I'd recommend replacing "net" with "network", as "net" is a somewhat
overloaded term.

[mmenke, 2013/07/25 03:39:18]

"net" specifically refers to src/net, and the "net" namespace.  It's seems more
accurate than network, despite the overloading.

+  </summary>
+</histogram>
+
+<histogram name="SBClientDownload.DownloadRequestResponseCode">
+  <summary>
+    For CheckClientDownloadRequest URLFetchers with successful status, the HTTP
+    response code that was received.
+  </summary>
+</histogram>
+
 <histogram name="SBClientDownload.SignedBinaryDownload"
     enum="SBClientDownloadIsSignedBinary">
   <summary>