Worker: Add UMA to record an exit code of WorkerThread

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 9005b9cb64b393e9a7876bb112a5f2d7677cc152..cd879d8ee9527118d656499566b2af2bbd83bb5c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -63592,6 +63592,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WorkerThread.ExitCode" enum="WorkerThreadExitCode">
+  <owner>nhiroki@chromium.org</owner>
+  <summary>Records the exit code of WorkerThread.</summary>
+</histogram>
+
 <histogram name="WrenchMenu.MenuAction" enum="WrenchMenuAction">
   <owner>ainslie@chromium.org</owner>
   <owner>edwardjung@chromium.org</owner>
@@ -91916,6 +91921,13 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="2" label="People"/>
 </enum>
 
+<enum name="WorkerThreadExitCode" type="int">
+  <int value="0" label="NotTerminated"/>
+  <int value="1" label="GracefullyTerminated"/>
+  <int value="2" label="SyncForciblyTerminated"/>
+  <int value="3" label="AsyncForciblyTerminated"/>
+</enum>
+
 <enum name="WrenchMenuAction" type="int">
   <int value="0" label="New tab"/>
   <int value="1" label="New window"/>