Add flow graph printing into a .cfg file with flag --print-flow-graph-file.

vm/os.h

Index: vm/os.h
===================================================================
--- vm/os.h	(revision 8145)
+++ vm/os.h	(working copy)
@@ -84,6 +84,7 @@
   // Print formatted output to stdout/stderr for debugging.
   static void Print(const char* format, ...);
   static void PrintErr(const char* format, ...);
+  static void FPrint(FILE* stream, const char* format, ...);
   static void VFPrint(FILE* stream, const char* format, va_list args);
   // Print formatted output info a buffer.
   // Does not write more than size characters (including the trailing '\0').
@@ -101,6 +102,8 @@
                       const char* format,
                       va_list args);
 
+  static FILE* FOpen(const char* path, const char* mode);
+
   // Initialize the OS class.
   static void InitOnce();