Add installer::SetupSingleton.

chrome/app/chromium_strings.grd

Index: chrome/app/chromium_strings.grd
diff --git a/chrome/app/chromium_strings.grd b/chrome/app/chromium_strings.grd
index 925b598cdbae91ebd16c3d977c34657056b2e28c..c52902cbc244bd11f201338a78f65da0e2400668 100644
--- a/chrome/app/chromium_strings.grd
+++ b/chrome/app/chromium_strings.grd
@@ -438,6 +438,9 @@ Chromium is unable to recover your settings.
         <message name="IDS_INSTALL_OS_ERROR" desc="Error displayed when any Windows API call fails and we do not have more specific information.">
           An operating system error occurred during installation. Please download Chromium again.
         </message>
+        <message name="IDS_INSTALL_SINGLETON_ACQUISITION_FAILED" desc="Error displayed when the installer cannot run because another installer is already running.">
+          Another operation on Chromium is in progress. Please try again later.
+        </message>
         <message name="IDS_INSTALL_TEMP_DIR_FAILED" desc="Error displayed when we fail to create temporary directory during installation.">
           The installer couldn't create a temporary directory. Please check for free disk space and permission to install software.
         </message>