Add UMA entries for web history UMA histograms.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index bd799a0e4cf00236e444701fc3212cade57d85bb..183df0095cab55c42a7c7256ae68c9a5ecd45a09 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -8990,6 +8990,21 @@ other types of suffix sets.
   </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
@@ -13859,6 +13874,12 @@ other types of suffix sets.
   <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"/>