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

src/v8utils.h

Index: src/v8utils.h
diff --git a/src/v8utils.h b/src/v8utils.h
index bb587e17332c4bbbe9248806097b36512009f2e0..38001e5f09e0acecd800c544cf22f83507d78e53 100644
--- a/src/v8utils.h
+++ b/src/v8utils.h
@@ -57,6 +57,9 @@ namespace internal {
 void PRINTF_CHECKING PrintF(const char* format, ...);
 void FPRINTF_CHECKING PrintF(FILE* out, const char* format, ...);
 
+// Prepredings the current process ID to the output.
+void PRINTF_CHECKING PrintPID(const char* format, ...);
+
 // Our version of fflush.
 void Flush(FILE* out);