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

src/platform.h

 // 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 299 matching lines @@
   static const int kMinComplexMemCopy = 64;
   typedef void (*MemCopyFunction)(void* dest, const void* src, size_t size);
 
 #else  // V8_TARGET_ARCH_IA32
   static void MemCopy(void* dest, const void* src, size_t size) {
     memcpy(dest, src, size);
   }
   static const int kMinComplexMemCopy = 256;
 #endif  // V8_TARGET_ARCH_IA32
 
+  static int GetCurrentProcessId();
+
  private:
   static const int msPerSecond = 1000;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(OS);
 };
 
 // Represents and controls an area of reserved memory.
 // Control of the reserved memory can be assigned to another VirtualMemory
 // object by assignment or copy-contructing. This removes the reserved memory
 // from the original object.
@@ skipping 427 matching lines @@
   Atomic32 active_;
   PlatformData* data_;  // Platform specific data.
   int samples_taken_;  // Counts stack samples taken.
   DISALLOW_IMPLICIT_CONSTRUCTORS(Sampler);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_PLATFORM_H_