Backed out changeset a4bf5887fc2a (bug 1680402) for being the most likely cause of bug 1686387. a=backout

mozglue/misc/Debug.h

@@ -45,9 +45,7 @@ inline void printf_stderr(const char* fmt, ...) MOZ_FORMAT_PRINTF(1, 2) {
   }
 #endif  // defined(XP_WIN)
 
-  // stderr is unbuffered by default so we open a new FILE (which is buffered)
-  // so that calls to printf_stderr are not as likely to get mixed together.
-  FILE* fp = _fdopen(_dup(_fileno(stderr)), "a");
+  FILE* fp = _fdopen(_dup(2), "a");
   if (!fp) return;
 
   va_list args;

xpcom/base/nsCRTGlue.cpp

@@ -253,9 +253,7 @@ void vprintf_stderr(const char* aFmt, va_list aArgs) {
     }
   }
 
-  // stderr is unbuffered by default so we open a new FILE (which is buffered)
-  // so that calls to printf_stderr are not as likely to get mixed together.
-  FILE* fp = _fdopen(_dup(_fileno(stderr)), "a");
+  FILE* fp = _fdopen(_dup(2), "a");
   if (!fp) {
     return;
   }