hosted: Add two histograms tracking "mayLaunchUrl" usage.

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 db98126919802b215a6b3f30debb60ab7243fd02..121b0de08a1a78a0b9cb3ca17aecf38d21c31a83 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -4403,6 +4403,22 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="CustomTabs.PredictionStatus" enum="PredictionStatus">
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    For Custom Tabs, records whether mayLaunchUrl() has been called, and if so,
+    whether the call was later matched by a URL launch.

[Ilya Sherman, 2015/06/03 22:07:32]

When exactly is this recorded?  I'm imagining that there might be some bias,
wherein "No prediction" is recorded immediately, but the other two are only
recorded after some -- potentially indefinite -- delay.  If so, that would bias
towards more "No prediction" records than others.

[Benoit L, 2015/06/04 11:16:29]

No, there is only one point in the code where this is recorded, and the
recording happens at the same point in time regardless of the value being
recorded.
Thank you for the comment, it is useful to keep it in mind for other metrics.

+  </summary>
+</histogram>
+
+<histogram name="CustomTabs.PredictionToLaunch" units="ms">

[Ilya Sherman, 2015/06/03 22:07:32]

It looks like you're introducing a new group name, "CustomTabs", in the
top-level namespace.  This might be appropriate, but please consider whether
there is an existing group name that makes sense to use.  If there isn't, please
consider whether the new group name strikes the right balance between being too
general and too specific, so that it carves off a reasonably sized chunk of the
namespace.

[Benoit L, 2015/06/04 11:16:29]

Thank you for the comment.
I looked at the top-level names in histogram, and none seem appropriate to me.
"Custom Tabs" is a new feature (formerly "ChromePlate") that will require more
histograms under it, and we want them separate in a group. It is
Android-specific though, but Android is not a top-level group (nor Mobile).

Does this work for you?

[Ilya Sherman, 2015/06/04 16:35:19]

Okay, fair enough.  Having a top-level group for a new feature seems reasonable.
 Thanks.

+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    When a URL prediction succeeds, time in ms between the prediction and the
+    actual launch.
+  </summary>
+</histogram>
+
 <histogram name="DataReductionProxy.BlockTypeFallback"
     enum="DataReductionProxyBypassType">
   <owner>bengr@chromium.org</owner>
@@ -61851,6 +61867,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="1" label="The pre-connect triggered host was accessed"/>
 </enum>
 
+<enum name="PredictionStatus" type="int">
+  <int value="0" label="No prediction"/>
+  <int value="1" label="Successful prediction"/>
+  <int value="2" label="Wrong prediction"/>
+</enum>
+
 <enum name="PrefetchStatus" type="int">
   <int value="0" label="undefined"/>
   <int value="1" label="success from cache"/>