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

tests/standalone/io/process_info_test.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "dart:io";
+
+import "package:expect/expect.dart";
+
+main() {
+  int currentRss = ProcessInfo.currentRss;
+  print('currentRss = $currentRss');
+  Expect.isTrue(currentRss > 0);
+
+  int maxRss = ProcessInfo.maxRss;
+  print('maxRss = $maxRss');
+  Expect.isTrue(maxRss > 0);
+
+  Expect.isTrue(currentRss <= maxRss);
+}