NTP: add histogram to track New Tab page URL status.

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..c7f423c214a408134691455083ff09574330ede0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6045,6 +6045,10 @@ other types of suffix sets.
 
 <histogram name="InstantExtended.FallbackToLocalOverlay"
     enum="InstantExtended_FallbackCause">
+  <obsolete>
+    Depcreated as of 10/2013. No longer relevant since the HTML overlay was
+    abandoned.
+  </obsolete>
   <summary>
     Records the cause for falling back to a local overlay at the time of
     fallback.
@@ -6063,6 +6067,14 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="InstantExtended.LocalNTPFallbackReason"
+    enum="InstantExtended_LocalNTPFallbackReason">
+  <summary>
+    Records a histogram for reasons for falling back to a local NTP instead of
+    using a valid search provider's URL.
+  </summary>
+</histogram>
+
 <histogram name="InstantExtended.NewOptInState"
     enum="InstantExtended_NewOptInState">
   <obsolete>
@@ -26392,6 +26404,12 @@ other types of suffix sets.
   <int value="4" label="Non-extended navigation"/>
 </enum>
 
+<enum name="InstantExtended_LocalNTPFallbackReason" type="int">
+  <int value="0" label="Insecure NTP URL"/>
+  <int value="1" label="Suggest disabled"/>
+  <int value="2" label="URL blocked for supervised user"/>
+</enum>
+
 <enum name="InstantExtended_NewOptInState" type="int">
   <int value="0" label="Default"/>
   <int value="1" label="Opted in"/>