Add installer::SetupSingleton.

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 f306e4efd6977bf52abe6738a3ef10dc6c9ab778..d2c083963bee956c7fd07d7050e98cd5707faf23 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -19768,6 +19768,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Installer.SetupSingletonAcquisitionResult"
+    enum="InstallerSetupSingletonAcquisitionResult">
+  <owner>fdoray@chromium.org</owner>
+  <summary>
+    The result of trying to acquire a setup singleton. On Windows, a setup.exe
+    process must hold the setup singleton of a Chrome installation when it makes
+    changes to it.
+  </summary>
+</histogram>
+
 <histogram name="Installer.SuccessfulMBsDownloadedFrom" units="MB">
   <owner>zeuthen@chromium.org</owner>
   <summary>
@@ -83534,6 +83544,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="35" label="Ran by google.com/intl/ (deprecated)"/>
 </enum>
 
+<enum name="InstallerSetupSingletonAcquisitionResult" type="int">
+  <int value="0" label="The setup singleton was acquired successfully."/>
+  <int value="1" label="Acquisition of the exit event mutex timed out."/>
+  <int value="2" label="Acquisition of the setup mutex timed out."/>
+</enum>
+
 <enum name="InstallStatus" type="int">
   <int value="0" label="FIRST_INSTALL_SUCCESS"/>
   <int value="1" label="INSTALL_REPAIRED"/>
@@ -83582,6 +83598,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="57" label="UNPACKING_FAILED"/>
   <int value="58" label="IN_USE_DOWNGRADE"/>
   <int value="59" label="OLD_VERSION_DOWNGRADE"/>
+  <int value="60" label="SETUP_SINGLETON_ACQUISITION_FAILED"/>
 </enum>
 
 <enum name="InstantControllerEvent" type="int">