Add histogram to measure the read deferral on crosswebsite navigation

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 6986 matching lines @@
   </summary>
 </histogram>
 
 <histogram name="Net.RenegotiationExtensionSupported">
   <summary>
     True if the HTTP request was sent to a server which supports the TLS
     renegotiation extension.
   </summary>
 </histogram>
 
+<histogram name="Net.ResourceLoader.ReadDeferral" units="milliseconds">
+  <summary>
+    When starting a cross-site navigation, the time between reading the headers
+    and body of the response.
+  </summary>
+</histogram>
+
 <histogram name="Net.SocketIdleTimeBeforeNextUse_ReusedSocket">
   <summary>The time an already used socket sat idle before being used.</summary>
 </histogram>
 
 <histogram name="Net.SocketIdleTimeBeforeNextUse_UnusedSocket">
   <summary>
     The time an unused socket (all HTTP sockets, regardless of any proxy used)
     sat idle before being used.
   </summary>
 </histogram>
@@ skipping 17322 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>