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

runtime/bin/process_fuchsia.cc

Index: runtime/bin/process_fuchsia.cc
diff --git a/runtime/bin/process_fuchsia.cc b/runtime/bin/process_fuchsia.cc
index 26ca132afe6f114a3f1663492827644b6c4fd530..11e21acc98f15ed6a5e2b175fb6c7969ed4afa95 100644
--- a/runtime/bin/process_fuchsia.cc
+++ b/runtime/bin/process_fuchsia.cc
@@ -431,6 +431,25 @@ intptr_t Process::CurrentProcessId() {
 }
 
 
+intptr_t Process::CurrentRSS() {
+  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);
+  if (status != NO_ERROR) {
+    return ERRNO(status);
+  }
+  return task_stats.mem_committed_bytes;
+}
+
+
+intptr_t Process::MaxRSS() {
+  // There is currently no way to get the high watermark value on Fuchsia, so
+  // just return the current RSS value.
+  return CurrentRSS();
+}
+
+
 static bool ProcessWaitCleanup(intptr_t out,
                                intptr_t err,
                                intptr_t exit_event,