Add histogram for image 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 992 matching lines @@
   </summary>
 </histogram>
 
 <histogram name="Net.DownloadBandwidth">
   <summary>
     Kbps on download streams exceeding 25KB.  Measures from the beginning of the
     first byte received until the end of flowing data.
   </summary>
 </histogram>
 
+<histogram name="Net.ErrorCodesForImages" enum="NetErrorCodes">
+  <summary>
+    Net error codes that requests for images end with, including net::OK and
+    net:ERR_ABORTED.
+  </summary>
+</histogram>
+
 <histogram name="Net.ErrorCodesForMainFrame" enum="NetErrorCodes">
   <obsolete>
     Deprecated as of 2011/5/24, replaced by Net.ErrorCodesForMainFrame2, which
     measures the same data but uses a different bucket structure (adds guard
     buckets).
   </obsolete>
   <summary>
     Positive net error code that a page failed with. Note that this only counts
     the errors in &quot;main frames&quot;, so it is a measure of the error pages
     that users actually see (it does not for example count the error codes for
@@ skipping 6387 matching lines @@
 <fieldtrial name="SSLResumption">
   <group name="Resume_Handshake" label="Session Resumption"/>
   <group name="Full_Handshake" label="Full"/>
   <affected-histogram name="Net.SSL_Connection_Latency"/>
   <affected-histogram name="Net.SSL_Connection_Latency_Google"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>