Provide a message to Google Update for the user when a system-level Chrome is already installed.

chrome/app/chromium_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 <!-- This file contains definitions of strings that are distribution specific.
 If you update this file, be sure also to update google_chrome_strings.grd. -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       output_all_resource_defines="false" source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/chromium_strings.h" type="rc_header">
       <emit emit_type='prepend'></emit>
     </output>
@@ skipping 438 matching lines @@
         </message>
         <message name="IDS_INSTALL_INSUFFICIENT_RIGHTS" desc="Error displayed when a non admin user tries to attempt system level install/uninstall.">
           You do not have appropriate rights for system-level install. Try running the installer again as Administrator.
         </message>
         <message name="IDS_INSTALL_NO_PRODUCTS_TO_UPDATE" desc="Error displayed if the installer is not provided one or more products to install/update.">
           No installation of Chromium found to update.
         </message>
         <message name="IDS_INSTALL_MULTI_INSTALLATION_EXISTS" desc="Error during install if a multi-install Chromium is present, thereby preventing the installation of a single-install Chromium.">
           A conflicting installation of Chromium was found on the system. Please uninstall it and try again.
         </message>
+        <message name="IDS_INSTALL_EXISTING_VERSION_LAUNCHED" desc="A message shown to users who try to install Chrome in their user profile directory when their computer already has Chrome installed for all users. In this case, the installer silently launches the existing version of Chrome for all users rather than installing a second version of Chrome.">
+          Chromium is already installed for all users on your computer.
+        </message>
       </if>
       <!-- Options Dialog -->
       <message name="IDS_OPTIONS_IMPROVE_BROWSING_EXPERIENCE" desc="The text in the options panel that describes how we use web services to improve browsing experience.">
         Chromium may use web services to improve your browsing experience.
       </message>
       <if expr="is_win">
         <message name="IDS_SHORTCUT_TOOLTIP" desc="Text for the hover-on tooltip for the Chromium shortcuts.">
           Access the Internet
         </message>
         <message name="IDS_UNINSTALL_DELETE_PROFILE" desc="Text to show user to ask whether to delete all the profile data also during uninstallation.">
@@ skipping 782 matching lines @@
         </message>
         <message name="IDS_METRO_FLOW_LOGO_STRING_ALT"
                  desc="Alt text for the faded Chrome logo image.">
           Chromium logo
         </message>
       </if>
 
     </messages>
   </release>
 </grit>