Print IDS_USED_EXISTING_BROWSER on all platforms

This string was landed as linux-only here:
https://codereview.chromium.org/155740

It was changed be !OS_MACOSX when OS_LINUX was changed to OS_POSIX.

Bug: 566496
Change-Id: I5615ae7e304c51d7f4f8dd37df36a3c4d2e7cfeb
Reviewed-on: https://chromium-review.googlesource.com/764607Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515857}

chrome/app/generated_resources.grd

@@ -8039,7 +8039,7 @@ Please help our engineers fix this problem. Tell us what happened right before y
         </message>
       </if>
 
-      <message name="IDS_USED_EXISTING_BROWSER" desc="Linux-specific message printed to console when the app is ran a second time, causing a new window to show rather than starting up a new browser.">
+      <message name="IDS_USED_EXISTING_BROWSER" desc="Message printed to console when the app is ran a second time, causing a new window to show rather than starting up a new browser.">
         Created new window in existing browser session.
       </message>
 

chrome/browser/chrome_browser_main.cc

@@ -1460,11 +1460,10 @@ int ChromeBrowserMainParts::PreMainMessageLoopRunImpl() {
       break;
 
     case ProcessSingleton::PROCESS_NOTIFIED:
-#if defined(OS_POSIX) && !defined(OS_MACOSX)
-      // On POSIX systems, print a message notifying the process is running.
-      printf("%s\n", base::SysWideToNativeMB(base::UTF16ToWide(
-          l10n_util::GetStringUTF16(IDS_USED_EXISTING_BROWSER))).c_str());
-#endif  // defined(OS_POSIX) && !defined(OS_MACOSX)
+      printf("%s\n", base::SysWideToNativeMB(
+                         base::UTF16ToWide(l10n_util::GetStringUTF16(
+                             IDS_USED_EXISTING_BROWSER)))
+                         .c_str());
 
       // Having a differentiated return type for testing allows for tests to
       // verify proper handling of some switches. When not testing, stick to