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

runtime/bin/process.h

Index: runtime/bin/process.h
diff --git a/runtime/bin/process.h b/runtime/bin/process.h
index 734db37c84103801a3a00fd67935c59896f35c90..683e747837c7124b24b40682b2fcd383854debd9 100644
--- a/runtime/bin/process.h
+++ b/runtime/bin/process.h
@@ -146,6 +146,9 @@ class Process {
                                              intptr_t* pid);
   static Dart_Handle SetProcessIdNativeField(Dart_Handle process, intptr_t pid);
 
+  static int64_t CurrentRSS();
+  static int64_t MaxRSS();
+
  private:
   static int global_exit_code_;
   static Mutex* global_exit_code_mutex_;