[Offline Pages] Adds UMA for result of attempts to immediately start background loading.

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 9a110f5473b30c6e43d8a18b3dfc4808feccf121..8bb75eb4150dc6904788816a85ae8ea4bf9b6a19 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -38342,6 +38342,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Result of servicing requests that may contain offline page.</summary>
 </histogram>
 
+<histogram name="OfflinePages.Background.ImmediateStartStatus"
+    enum="OfflinePagesBackgroundImmediateStartStatus">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Status code of attempts to immediately starting background loading.
+  </summary>
+</histogram>
+
 <histogram name="OfflinePages.Background.OfflinerRequestStatus"
     enum="OfflinePagesBackgroundOfflinerRequestStatus">
   <owner>dougarnett@chromium.org</owner>
@@ -91412,6 +91420,15 @@ value.
   <int value="11" label="Unknown net error, showing error page"/>
 </enum>
 
+<enum name="OfflinePagesBackgroundImmediateStartStatus" type="int">
+  <int value="0" label="Busy"/>
+  <int value="1" label="Started"/>
+  <int value="2" label="Not accepted by loader"/>
+  <int value="3" label="No connection"/>
+  <int value="4" label="No effective connection"/>
+  <int value="5" label="Not started on svelte device"/>
+</enum>
+
 <enum name="OfflinePagesBackgroundOfflinerRequestStatus" type="int">
 <!-- Generated from components/offline_pages/background/offliner.h -->