Add UMA stats for Scout initialization logic

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 487b1ab170bdf8d93c9dc5d8790e6f9a72a2755e..121af831a23468e7ceaedfd77f7d915ae881f74d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -53290,6 +53290,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SafeBrowsing.Pref.Scout.Transition"
+    enum="ScoutTransitionReason">
+  <owner>lpz@chromium.org</owner>
+  <summary>
+    Tracks reasons for the Extended Reporting preference transition, such as a
+    user entering an experiment group or seeing a security interstitial for the
+    first time. Recorded for all non-Incognito profiles on profile startup.
+  </summary>
+</histogram>
+
 <histogram name="SafeBrowsing.UnverifiedDownloads.Allowed"
     enum="SBClientDownloadExtensions">
   <owner>asanka@chromium.org</owner>
@@ -100969,6 +100979,24 @@ value.
   <int value="11" label="(gesture) ScheduledPageBlock"/>
 </enum>
 
+<enum name="ScoutTransitionReason" type="int">
+  <int value="0" label="Flag forced Scout Group to true"/>
+  <int value="1" label="Flag forced Scout Group to false"/>
+  <int value="2" label="User in OnlyShowScout group, enters Scout Group"/>
+  <int value="3"
+      label="User in CanShowScout group, enters Scout Group immediately"/>
+  <int value="4"
+      label="User in CanShowScout group, waiting for interstitial to enter
+             Scout Group"/>
+  <int value="5"
+      label="User in CanShowScout group saw first interstitial and entered
+             Scout Group"/>
+  <int value="6" label="User in Control group"/>
+  <int value="7" label="Rollback: SBER2 on on implies SBER1 can turn on"/>
+  <int value="8" label="Rollback: SBER2 off so SBER1 must be turned off"/>
+  <int value="9" label="Rollback: SBER2 absent so SBER1 must be cleared"/>
+</enum>
+
 <enum name="ScrollThread" type="int">
   <int value="0" label="Scroll on impl-thread"/>
   <int value="1" label="Scroll on main-thread"/>