Split the NullInitialized signal out of TrustedInitialized.

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 e225050bcaa860786c5c188d42f6de4ebb4f4f8f..f43aacc89e80967216fe6cb0158324105293fc0b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29566,6 +29566,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Settings.TrackedPreferenceNullInitialized"
+    enum="TrackedPreference">
+  <owner>gab@chromium.org</owner>
+  <summary>
+    The id of a tracked preference which was initialized despite the absence of
+    a MAC as its value was NULL.
+  </summary>
+</histogram>
+
 <histogram name="Settings.TrackedPreferenceReset" enum="TrackedPreference">
   <owner>gab@chromium.org</owner>
   <summary>The id of a tracked preference which was reset by Chrome.</summary>
@@ -29625,8 +29634,8 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <owner>gab@chromium.org</owner>
   <summary>
     The id of a tracked preference which was initialized despite the absence of
-    a MAC as either (1) the current MACs are trusted, infering that this is a
-    newly tracked pref, or (2) its value is NULL.
+    a MAC as the current MACs are trusted, infering that this is a newly tracked
+    pref.
   </summary>
 </histogram>