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

src/v8utils.cc

Index: src/v8utils.cc
diff --git a/src/v8utils.cc b/src/v8utils.cc
index 042a60f0b41ada57a9ababe8f8970497d749e3b4..627169e7090362a601b1ff713a0861083e89fb3a 100644
--- a/src/v8utils.cc
+++ b/src/v8utils.cc
@@ -53,6 +53,15 @@ void PrintF(FILE* out, const char* format, ...) {
 }
 
 
+void PrintPID(const char* format, ...) {
+  OS::Print("[%d] ", OS::GetCurrentProcessId());
+  va_list arguments;
+  va_start(arguments, format);
+  OS::VPrint(format, arguments);
+  va_end(arguments);
+}
+
+
 void Flush(FILE* out) {
   fflush(out);
 }