One-time reset of UMA client ids and low entropy source.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
===================================================================
--- tools/metrics/histograms/histograms.xml	(revision 260135)
+++ tools/metrics/histograms/histograms.xml	(working copy)
@@ -27656,6 +27656,14 @@
   <summary>Whether the scroll is executed on main thread.</summary>
 </histogram>
 
+<histogram name="UMA.ClientIdMigrated" enum="BooleanMigrated">
+  <owner>asvitkine@chromium.org</owner>
+  <summary>
+    Recorded when the one-time UMA client id reset was performed (and the client
+    id of this user was migrated).
+  </summary>
+</histogram>
+
 <histogram name="UMA.CollectExternalEventsTime" units="milliseconds">
   <owner>asvitkine@chromium.org</owner>
   <summary>
@@ -29933,6 +29941,11 @@
   <int value="1" label="Matched"/>
 </enum>
 
+<enum name="BooleanMigrated" type="int">
+  <int value="0" label="Not migrated"/>
+  <int value="1" label="Migrated"/>
+</enum>
+
 <enum name="BooleanOrphan" type="int">
   <int value="0" label="Non-orphan"/>
   <int value="1" label="Orphan"/>