Add detailed histograms for DownloadProtectionService URLFetcher errors.

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 12399 matching lines @@
 </histogram>
 
 <histogram name="SBClientDownload.DownloadRequestDuration" units="milliseconds">
   <summary>
     Records the total time it takes for the SafeBrowsing download service to
     check whether the content of a download is malicious or not. This histogram
     only includes requests that are sent to the SafeBrowsing server.
   </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>
     Records the number of signed vs. unsigned executables that are downloaded.
   </summary>
 </histogram>
 
 <histogram name="SBClientDownload.SignedOrWhitelistedDownload">
   <summary>
     Counter which is incremented whenever an executable is downloaded which is
@@ skipping 11019 matching lines @@
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>
   <group name="PlusIcon" label="Plus icon in apps page"/>
   <affected-histogram name="Extensions.AppLaunch"/>
   <affected-histogram name="NewTabPage.DefaultPageType"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>