Use process_singleton_linux on Mac.

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 f9898894bc9c3b5ad37c76117cf479dc9f21637f..bb335a59a6cd302190075839d5d9c4eba03644b9 100644
--- a/chrome/app/chromium_strings.grd
+++ b/chrome/app/chromium_strings.grd
@@ -1093,8 +1093,8 @@ Signing in anyway will merge Chromium information like bookmarks, history, and o
       </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 Chromium process (<ph name="PROCESS_ID">$1<ex>12345</ex></ph>) on another computer (<ph name="HOST_NAME">$2<ex>example.com</ex></ph>). Chromium 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 Chromium.
         </message>
       </if>