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

src/incremental-marking.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 197 matching lines @@
   MarkingDeque* marking_deque() { return &marking_deque_; }
 
   bool IsCompacting() { return IsMarking() && is_compacting_; }
 
   void ActivateGeneratedStub(Code* stub);
 
   void NotifyOfHighPromotionRate() {
     if (IsMarking()) {
       if (allocation_marking_factor_ < kFastMarking) {
         if (FLAG_trace_gc) {
-          PrintF("Increasing marking speed to %d due to high promotion rate\n",
-                 static_cast<int>(kFastMarking));
+          PrintPID("Increasing marking speed to %d "
+                   "due to high promotion rate\n",
+                   static_cast<int>(kFastMarking));
         }
         allocation_marking_factor_ = kFastMarking;
       }
     }
   }
 
   void EnterNoMarkingScope() {
     no_marking_scope_depth_++;
   }
 
@@ skipping 54 matching lines @@
   intptr_t allocated_;
 
   int no_marking_scope_depth_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(IncrementalMarking);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_INCREMENTAL_MARKING_H_