Add OS::GetCurrentProcessId and prepend output from trace-gc with the current pid

src/platform-posix.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 134 matching lines @@
 UNARY_MATH_FUNCTION(sqrt, CreateSqrtFunction())
 
 #undef MATH_FUNCTION
 
 
 double OS::nan_value() {
   // NAN from math.h is defined in C99 and not in POSIX.
   return NAN;
 }
 
+int OS::GetCurrentProcessId() {
+  return static_cast<int>(getpid());
+}
 
 // ----------------------------------------------------------------------------
 // POSIX date/time support.
 //
 
 int OS::GetUserTime(uint32_t* secs,  uint32_t* usecs) {
   struct rusage usage;
 
   if (getrusage(RUSAGE_SELF, &usage) < 0) return -1;
   *secs = usage.ru_utime.tv_sec;
@@ skipping 371 matching lines @@
   return ntohl(value);
 }
 
 
 Socket* OS::CreateSocket() {
   return new POSIXSocket();
 }
 
 
 } }  // namespace v8::internal