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

src/incremental-marking-inl.h

Index: src/incremental-marking-inl.h
diff --git a/src/incremental-marking-inl.h b/src/incremental-marking-inl.h
index 2dae6f207d0a3904408c052e791e638d6ebf7f4f..5c2c8ab43147825dd13c4cdbd38b6b5659f80a12 100644
--- a/src/incremental-marking-inl.h
+++ b/src/incremental-marking-inl.h
@@ -107,7 +107,7 @@ void IncrementalMarking::BlackToGreyAndUnshift(HeapObject* obj,
       // trace it.  In this case we switch to non-incremental marking in
       // order to finish off this marking phase.
       if (FLAG_trace_gc) {
-        PrintF("Hurrying incremental marking because of lack of progress\n");
+        PrintPID("Hurrying incremental marking because of lack of progress\n");
       }
       allocation_marking_factor_ = kMaxAllocationMarkingFactor;
     }