Use process_singleton_linux on Mac.

chrome/app/chromium_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 <!-- This file contains definitions of strings that are distribution specific.
 These strings will not be translated and only english versions will
 be available for now. -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/chromium_strings.h" type="rc_header">
       <emit emit_type='prepend'></emit>
@@ skipping 1086 matching lines @@
         Chromium is using your camera and microphone.
       </message>
       <message name="IDS_MEDIA_STREAM_STATUS_TRAY_TEXT_AUDIO_ONLY" desc="Tool tip for the capture status tray icon when microphone is being used">
         Chromium is using your microphone.
       </message>
       <message name="IDS_MEDIA_STREAM_STATUS_TRAY_TEXT_VIDEO_ONLY" desc="Tool tip for the capture status tray icon when camera is being used">
         Chromium is using your camera.
       </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>
 
       <!-- Mac First-run dialog messages -->
       <if expr="is_macosx">
         <message name="IDS_FIRSTRUN_DLG_MAC_SET_DEFAULT_BROWSER_LABEL" desc="Label for checkbox that sets the default browser">
           Set Chromium as your default browser
         </message>
       </if>
@@ skipping 84 matching lines @@
         <message name="IDS_NEW_TAB_OTR_EXTENSIONS_MESSAGE"
                  desc="Explanation in a new OTR window that extensions have been disabled while in OTR.">
           Because Chromium does not control how extensions handle your personal data, all extensions have been disabled for incognito windows. You can reenable them individually in the
           <ph name="BEGIN_LINK">&lt;a href="$1"&gt;</ph>extensions manager<ph name="END_LINK">&lt;/a&gt;<ex>&lt;/a&gt;</ex></ph>.
         </message>
       </if>
 
     </messages>
   </release>
 </grit>