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 41da607a2a839b3ac49e765536be5bbe99f622d7..c3da9c8e47053e6520e6d7a701ea9f07cb0f33d3 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -53066,6 +53066,16 @@ 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

[Nathan Parker, 2016/12/03 18:06:36]

So is this recorded only when a state transition occurs?  like, if I restart
twice in a row, chances are the second time there'll be no change?

This seems pretty useful for seeing how users are moving.  Cool

[lpz, 2016/12/05 16:37:58]

yep, that's correct, only on transition. The exception is the one called
"control" which will be incremented on each startup for users in that group. But
control also has its own transitions for rollbacks etc that will only be
recorded once.

+    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>
@@ -100330,8 +100340,6 @@ value.
   <int value="193" label="PUP"/>
   <int value="194" label="QUAD"/>
   <int value="195" label="R00"/>
-  <int value="196" label="R01"/>

[Nathan Parker, 2016/12/03 18:06:37]

This looks like an unintentional change... maybe need to rebase?

[lpz, 2016/12/05 16:37:58]

Done.

-  <int value="197" label="R02"/>
   <int value="198" label="R03"/>
   <int value="199" label="R04"/>
   <int value="200" label="R05"/>
@@ -100515,6 +100523,21 @@ 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 Control group"/>
+  <int value="6" label="Rollback: SBER2 on on implies SBER1 can turn on"/>
+  <int value="7" label="Rollback: SBER2 off so SBER1 must be turned off"/>
+  <int value="8" 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"/>