Allow setting thread priorities in Chromium on all but linux platforms.
The previous check was overly broad, so narrowing it down to linux only.

R=pbos@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/43929004

Cr-Commit-Position: refs/heads/master@{#8837}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8837 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/system_wrappers/source/thread_posix.cc b/webrtc/system_wrappers/source/thread_posix.cc
index e110af5..e326a29 100644
--- a/webrtc/system_wrappers/source/thread_posix.cc
+++ b/webrtc/system_wrappers/source/thread_posix.cc
@@ -115,7 +115,7 @@
   DCHECK(thread_checker_.CalledOnValidThread());
   if (!thread_)
     return false;
-#if defined(WEBRTC_CHROMIUM_BUILD)
+#if defined(WEBRTC_CHROMIUM_BUILD) && defined(WEBRTC_LINUX)
   // TODO(tommi): Switch to the same mechanism as Chromium uses for
   // changing thread priorities.
   return true;
@@ -145,7 +145,7 @@
   }
 
   return true;
-#endif  // defined(WEBRTC_CHROMIUM_BUILD)
+#endif  // defined(WEBRTC_CHROMIUM_BUILD) && defined(WEBRTC_LINUX)
 }
 
 void ThreadPosix::Run() {