Fix get_cpu_profile_timeline_rpc_test.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 36ee8097e22ef8531f5f0adafcb6b5d98523aa4a..c698ffb8238b9e139c3b87a2297d98c6b5a7822b 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -3242,7 +3242,7 @@ void Service::PrintJSONForVM(JSONStream* js, bool ref) {
   jsobj.AddProperty("targetCPU", CPU::Id());
   jsobj.AddProperty("hostCPU", HostCPUFeatures::hardware());
   jsobj.AddProperty("version", Version::String());
-  jsobj.AddProperty("pid", OS::ProcessId());
+  jsobj.AddProperty64("pid", OS::ProcessId());
   int64_t start_time_millis = (vm_isolate->start_time() /
                                kMicrosecondsPerMillisecond);
   jsobj.AddPropertyTimeMillis("startTime", start_time_millis);