[Background fetching] Configure and report fetching triggers

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 491792d47df87a0a287ae918a252861266a8e43d..c963b38c1d6674289fb8c26f167ac772c64c9bc1 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -38780,6 +38780,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.ContentSuggestions.BackgroundFetchTrigger"
+    enum="BackgroundFetchTrigger">
+  <owner>jkrcal@chromium.org</owner>
+  <summary>
+    Android: The type of trigger that caused a background fetch of NTP content
+    suggestions from a suggestion server. Every background fetch is recorded.
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.ContentSuggestions.CategoryDismissed"
     enum="ContentSuggestionsCategory">
   <owner>treib@chromium.org</owner>
@@ -77744,6 +77753,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="Ok, got it"/>
 </enum>
 
+<enum name="BackgroundFetchTrigger" type="int">
+  <int value="0" label="Wake-up of the persistent scheduler"/>
+  <int value="1" label="NTP opened"/>
+  <int value="2" label="Browser foregrounded"/>
+  <int value="3" label="Cold start of the browser"/>
+</enum>
+
 <enum name="BackgroundModeMenuItem" type="int">
   <int value="0" label="About"/>
   <int value="1" label="Task manager"/>