Avoid crashing the browser on truncated reads from the cache backend

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 808 matching lines @@
     The time spent waiting for write lock on a disk cache entry.
   </summary>
 </histogram>
 
 <histogram name="HttpCache.OfflineStatus" enum="OfflineStatus">
   <summary>
     Result of a main page HttpCacheTransaction if offline mode had been enabled.
   </summary>
 </histogram>
 
+<histogram name="HttpCache.TruncatedHeader" units="BooleanTruncated">
+  <summary>Whether cache headers were truncated before read.</summary>
+</histogram>
+
+<histogram name="HttpCache.TruncatedMetadata" units="BooleanTruncated">
+  <summary>Whether cache entry metadata was truncated before read.</summary>
+</histogram>
+
 <histogram name="Import.ImporterType.AutoImport" enum="ImporterType">
   <summary>The importer used on first run Auto Import.</summary>
 </histogram>
 
 <histogram name="Import.ImporterType.BookmarksAPI" enum="ImporterType">
   <summary>The importer used on import from the bookmarks file API.</summary>
 </histogram>
 
 <histogram name="Import.ImporterType.ImportDataHandler" enum="ImporterType">
   <summary>
@@ skipping 5535 matching lines @@
 <enum name="BooleanSuccess" type="int">
   <int value="0" label="Failure"/>
   <int value="1" label="Success"/>
 </enum>
 
 <enum name="BooleanTabDiscard" type="int">
   <int value="0" label="Memory OK, no discards"/>
   <int value="1" label="Memory low, tabs discarded"/>
 </enum>
 
+<enum name="BooleanTruncated" type="int">
+  <int value="0" label="Was Not Truncated"/>
+  <int value="1" label="Was Truncated"/>
+</enum>
+
 <enum name="BooleanUsage" type="int">
   <int value="0" label="Not Used"/>
   <int value="1" label="Used"/>
 </enum>
 
 <enum name="BooleanWiped" type="int">
   <int value="0" label="Re-enabled"/>
   <int value="1" label="Wiped out"/>
 </enum>
 
@@ skipping 4181 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>