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 d1a1a1030dc9e6a44c3401bee9e6867d3cf3900a..8032e018d7de4afa0d034caeaf5157bf1bb516fe 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17553,6 +17553,13 @@ 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
@@ -34684,6 +34691,11 @@ 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"/>