Add histogram for safebrowsing download ping payload size.

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 12525 matching lines @@
     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.
   </summary>
 </histogram>
 
+<histogram name="SBClientDownload.DownloadRequestPayloadSize" units="bytes">
+  <summary>
+    The size of the upload data for CheckClientDownloadRequest URLFetchers.
+  </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>
@@ skipping 11182 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>