Add histograms for MouseEventPrefetch.Mouse*

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 1066 matching lines @@
 
 <histogram name="MemoryAndroid.NotificationForeground"
     enum="AndroidMemoryNotificationForeground">
   <summary>
     Memory notifications delivered through system callbacks to Chrome while in
     the foreground - we count LowMemory notification vs particular levels of
     TrimMemory foreground notification.
   </summary>
 </histogram>
 
+<histogram name="MouseEventPrefetch.MouseDownDuration_Click" units="ms">
+  <summary>
+    Measures the time elapsed between when the user mousedown-ed a link and when
+    the user clicked a link.
+  </summary>
+</histogram>
+
+<histogram name="MouseEventPrefetch.MouseDownFollowedByClick" enum="BooleanHit">
+  <summary>
+    Hits: The number of mousedown events detected prior to HTML anchor-tag
+    links' default click event handler. Miss: The number of HTML anchor-tag
+    links' default click events without prior mousedown events.

[Ilya Sherman, 2013/05/07 02:41:10]

It seems like you'd derive some benefit by creating a custom enumeration rather
than trying to recycle BooleanHit here.

[kouhei (in TOK), 2013/05/07 03:37:03]

Done.

+  </summary>
+</histogram>
+
+<histogram name="MouseEventPrefetch.MouseDowns">
+  <summary>
+    The number of mousedown events detected at HTML anchor-tag links' default
+    event handler.

[Ilya Sherman, 2013/05/07 02:41:10]

Does this histogram just have a single bucket?  If so, how do you plan to
interpret the numbers?  For example, suppose you see a billion mouse-downs
reported in a day.  How will that number be actionable?

[kouhei (in TOK), 2013/05/07 03:37:03]

The number will be compared to the hits of MouseOverFollowedByClick. The ratio
of MouseOver events followed by click over total num. of MouseOver events can be
calculated.

+  </summary>
+</histogram>
+
+<histogram name="MouseEventPrefetch.MouseOverDuration_Click" units="ms">
+  <summary>
+    Measures the time elapsed between when the user mouseover-ed a link and when
+    the user clicked a link.
+  </summary>
+</histogram>
+
+<histogram name="MouseEventPrefetch.MouseOverDuration_NoClick" units="ms">
+  <summary>
+    Measures the time elapsed between when the user mouseover-ed a link and when
+    the user mouseout-ed a link without click.
+  </summary>
+</histogram>
+
+<histogram name="MouseEventPrefetch.MouseOvers">
+  <summary>
+    The number of mouseover events detected at HTML anchor-tag links' default
+    event handler.
+  </summary>
+</histogram>
+
 <histogram name="Navigation.MainFrameScheme" enum="NavigationScheme">
   <summary>The scheme of the URL for each main-frame navigation.</summary>
 </histogram>
 
 <histogram name="Net.AsyncResourceHandler_PendingDataCount">
   <summary>
     The count of unacknowledged ResourceMsg_DataReceived messages.  This message
     is sent once per chunk of data read from the network.
   </summary>
 </histogram>
@@ skipping 10886 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>