Add histograms for MouseEventPrefetch.Mouse*

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 535d3bcc75c299c05a279f2200f158f49038f860..321388820ca759fec86a2a8b0ec715e00d65cfde 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1084,6 +1084,49 @@ other types of suffix sets.
   </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>