[dart:io] Adds ProcessInfo.{max,current}Rss. Adds OS::MaxRSS on Fuchsia.

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 209c825e5f1056a4e2892de27fe6060c99a9e9f1..74e965666f1aaa45b3f054d125779d3b905dd554 100644
--- a/runtime/vm/os_fuchsia.cc
+++ b/runtime/vm/os_fuchsia.cc
@@ -8,7 +8,9 @@
 #include "vm/os.h"
 
 #include <errno.h>
+#include <magenta/process.h>
 #include <magenta/syscalls.h>
+#include <magenta/syscalls/object.h>
 #include <magenta/types.h>
 
 #include "platform/assert.h"
@@ -156,8 +158,11 @@ int OS::NumberOfAvailableProcessors() {
 
 
 uintptr_t OS::MaxRSS() {
-  // TODO(US-95): Implement.
-  return 0;
+  mx_info_task_stats_t task_stats;
+  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;
 }