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 089cef7f691d40b1321588cdf1cf9f747e5e4d88..4a23884da96bbbbf0d768d2a596900247d2e6a55 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17428,6 +17428,13 @@ 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
@@ -34274,6 +34281,11 @@ 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"/>