[Fuchsia] Calculate RSS using new task stat fields

runtime/vm/os_fuchsia.cc

Index: runtime/vm/os_fuchsia.cc
diff --git a/runtime/vm/os_fuchsia.cc b/runtime/vm/os_fuchsia.cc
index efa8c892b710d775ec50a6a45c3087d3a3135f51..89c08fbc4f469c42124161c0f5c2fa8feae79ba2 100644
--- a/runtime/vm/os_fuchsia.cc
+++ b/runtime/vm/os_fuchsia.cc
@@ -156,7 +156,9 @@ uintptr_t OS::MaxRSS() {
   mx_handle_t process = mx_process_self();
   mx_status_t status = mx_object_get_info(
       process, MX_INFO_TASK_STATS, &task_stats, sizeof(task_stats), NULL, NULL);
-  return (status == NO_ERROR) ? task_stats.mem_committed_bytes : 0;
+  return (status == NO_ERROR)
+             ? (task_stats.mem_private_bytes + task_stats.mem_shared_bytes)
+             : 0;
 }