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

src/incremental-marking.h

Index: src/incremental-marking.h
diff --git a/src/incremental-marking.h b/src/incremental-marking.h
index 39e8daed68b04aed950debed6df6d382e84263cd..a9c2321d633e4bdee2e13f08df428612ca248ec1 100644
--- a/src/incremental-marking.h
+++ b/src/incremental-marking.h
@@ -215,8 +215,9 @@ class IncrementalMarking {
     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;
       }