Add UMA entries for web history UMA histograms.

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 8972 matching lines @@
   </summary>
 </histogram>
 
 <histogram name="WebFont.LocalFontUsed" enum="BooleanUsage">
   <summary>
     Whether a locallly installed font is actually used when @font-face had local
     sources.
   </summary>
 </histogram>
 
+<histogram name="WebHistory.LocalResultsMissingOnServer" units="%">
+  <summary>
+    Percentage of results that are present locally but are not returned by the
+    web history.

[Alexei Svitkine (slow), 2013/05/24 17:58:02]

Please mention how this is sampled. Is it when a page is opened, periodically,
etc?

[Sergiu, 2013/05/27 08:27:44]

Done.

+  </summary>
+</histogram>
+
+<histogram name="WebHistory.QueryCompletion" enum="WebHistoryStatus">
+  <summary>Whether the web history query was successful.</summary>

[Alexei Svitkine (slow), 2013/05/24 17:58:02]

Please add a little bit more context in the comment - what is the web history
query? Is it something that runs when you go to chrome://history? Or is it
related to the omnibox history provider, or ...?

The expanded comment should also make it clear when this is sampled.

Please do the same for the histogram below.

Thanks!

[Sergiu, 2013/05/27 08:27:44]

Done, tell me what you think now.

+</histogram>
+
+<histogram name="WebHistory.ResponseTime" units="milliseconds">
+  <summary>Time it took for the web history to reply.</summary>
+</histogram>
+
 <histogram name="WinTimeTicks.FailedToChangeCores" enum="WindowsVersion">
   <summary>
     Incremented each time the TimeTicks field trial runs on a machine with
     multiple cores, but failed to change thread affinity. Broken down by Windows
     version.
   </summary>
 </histogram>
 
 <histogram name="WinTimeTicks.MinResolutionNanoseconds" units="nanoseconds">
   <summary>
@@ skipping 4849 matching lines @@
   <int value="4" label="Set up Wallet"/>
   <int value="5" label="Accept ToS"/>
   <int value="6" label="Update expiration date"/>
   <int value="7" label="Upgrade min address"/>
   <int value="8" label="Choose another instrument or address"/>
   <int value="9" label="Verify CVV"/>
   <int value="10" label="Invalid form field"/>
   <int value="11" label="Require phone number"/>
 </enum>
 
+<enum name="WebHistoryStatus" type="int">
+  <int value="0" label="WEB_HISTORY_QUERY_FAILED">Failed</int>
+  <int value="1" label="WEB_HISTORY_QUERY_SUCCEEDED">Succeeded</int>
+  <int value="2" label="WEB_HISTORY_QUERY_TIMED_OUT">Timed out</int>
+</enum>
+
 <enum name="WebSocketHandshakeResult" type="int">
   <int value="0" label="Incomplete"/>
   <int value="1" label="Normal"/>
   <int value="2" label="Failed"/>
   <int value="3" label="Connected"/>
 </enum>
 
 <enum name="WebSocketPerMessageDeflateContextTakeOverMode" type="int">
   <int value="0" label="DoNotTakeOverContext"/>
   <int value="1" label="TakeOverContext"/>
@@ skipping 1505 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>