Multi-strategy based tracking.

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 85f75acfb923631e4a111788ebe1d8c49f2a77d4..e84d1dc4bd0c78219258ceae684e72f5b76e817d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17617,6 +17617,13 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Settings.TrackedSplitPreferenceChangedCount">
+  <summary>
+    The number of items that had changed in a dictionary pref when
+    Settings.TrackedPreferenceChanged is reported for that pref.
+  </summary>
+</histogram>
+
 <histogram name="SettingsResetBubble.NumNoThanksPerReset">
   <summary>
     Counts the number of times the user clicked on the No Thanks button of the
@@ -34803,6 +34810,11 @@ other types of suffix sets.
   <affected-histogram name="Startup.BrowserOpenTabs"/>
 </fieldtrial>
 
+<fieldtrial name="TrackedSplitPreferences" separator=".">
+  <group name="extension.settings" label="Extension IDs dictionary"/>
+  <affected-histogram name="Settings.TrackedSplitPreferenceChangedCount"/>
+</fieldtrial>
+
 <fieldtrial name="WebStoreLinkExperiment">
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>