Add UMA for window.external.AddSearchProvider (Chrome side).

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 c8d1c27bc67f6efb935ec1d7f9686da39e5e6672..3486c8cb71de08298f7ae142865340b3d0221cc5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40685,6 +40685,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Search.AddSearchProvider" enum="AddSearchProvider">
+  <owner>estade@chromium.org</owner>
+  <summary>
+    Measures usage of window.external.AddSearchProvider and user interaction
+    with the resulting confirmation dialog (if any).
+  </summary>
+</histogram>
+
 <histogram name="Search.DefaultSearchChangeOrigin"
     enum="DefaultSearchChangeOrigin">
   <owner>mathp@chromium.org</owner>
@@ -52262,6 +52270,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="IPv6"/>
 </enum>
 
+<enum name="AddSearchProvider" type="int">
+  <int value="0" label="window.external.AddSearchProvider was called"/>
+  <int value="1" label="A confirmation dialog was shown"/>
+  <int value="2" label="The user added a search engine via the dialog"/>
+  <int value="3" label="The user saw the dialog but didn't add an engine"/>

[Mark P, 2015/10/19 21:53:43]

but didn't add an engine
->
can you instead say something about cancelling or dismissing the window?

(For example, if the user sees the window and the chrome loses power, we don't
record the histogram even though the current label implies we should emit a
value of 3 to it.)

[Evan Stade, 2015/10/19 22:55:15]

Done.

+</enum>
+
 <enum name="AffiliationFetchResult" type="int">
   <int value="0" label="Success"/>
   <int value="1" label="Network/server error"/>