Make NS_IsMainThread faster on our major platforms. (Bug 521750) r=dbaron

xpcom/threads/nsThreadManager.cpp

@@ -38,11 +38,16 @@
 
 #include "nsThreadManager.h"
 #include "nsThread.h"
+#include "nsThreadUtils.h"
 #include "nsIClassInfoImpl.h"
 #include "nsTArray.h"
 #include "nsAutoPtr.h"
 #include "nsAutoLock.h"
 
+#ifdef NS_TLS
+NS_TLS bool gTLSIsMainThread = false;
+#endif
+
 typedef nsTArray< nsRefPtr<nsThread> > nsThreadArray;
 
 //-----------------------------------------------------------------------------
@@ -101,6 +106,10 @@ nsThreadManager::Init()
   // GetIsMainThread calls that occur post-Shutdown.
   mMainThread->GetPRThread(&mMainPRThread);
 
+#ifdef NS_TLS
+  gTLSIsMainThread = true;
+#endif
+
   mInitialized = PR_TRUE;
   return NS_OK;
 }