Use process_singleton_linux on Mac.

chrome/app/google_chrome_strings.grd

Index: chrome/app/google_chrome_strings.grd
diff --git a/chrome/app/google_chrome_strings.grd b/chrome/app/google_chrome_strings.grd
index 4e66d37bbbf03177fd070c249d51dd43c6e5941f..2acf82af0c485da66a185392c27bff308bb6809f 100644
--- a/chrome/app/google_chrome_strings.grd
+++ b/chrome/app/google_chrome_strings.grd
@@ -1017,8 +1017,8 @@ Signing in anyway will merge Chrome information like bookmarks, history, and oth
       </message>
 
       <!-- ProcessSingleton -->
-      <if expr="is_linux">
-        <message name="IDS_PROFILE_IN_USE_LINUX" desc="Message shown when the browser cannot start because the profile is in use on a different host.">
+      <if expr="is_linux or is_macosx">
+        <message name="IDS_PROFILE_IN_USE_POSIX" desc="Message shown when the browser cannot start because the profile is in use on a different host.">
           The profile appears to be in use by another Google Chrome process (<ph name="PROCESS_ID">$1<ex>12345</ex></ph>) on another computer (<ph name="HOST_NAME">$2<ex>example.com</ex></ph>).  Chrome has locked the profile so that it doesn't get corrupted.  If you are sure no other processes are using this profile, you can unlock the profile and relaunch Chrome.
         </message>
       </if>