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 5fc310d1d134336aebc33528ac18a0a110e095b9..2fa5eb4785908265c30c972840c3b43a319d7b44 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40791,6 +40791,14 @@ 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>
@@ -52385,6 +52393,14 @@ 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 closed it without adding an engine"/>
+</enum>
+
 <enum name="AffiliationFetchResult" type="int">
   <int value="0" label="Success"/>
   <int value="1" label="Network/server error"/>