fuchsia: Stub ProcessMetrics::GetWorkingSetKBytes

Referenced in some configs that we weren't trying to build before.

Bug: 706592
Change-Id: Ieae810b39ef6c99a2aedbda8db12763e81ff692a
Reviewed-on: https://chromium-review.googlesource.com/724422Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509626}

base/process/process_metrics_fuchsia.cc

@@ -39,6 +39,11 @@ size_t ProcessMetrics::GetPeakWorkingSetSize() const {
   return 0;
 }
 
+bool ProcessMetrics::GetWorkingSetKBytes(WorkingSetKBytes* ws_usage) const {
+  NOTIMPLEMENTED();  // TODO(fuchsia): https://crbug.com/706592.
+  return false;
+}
+
 bool GetSystemMemoryInfo(SystemMemoryInfoKB* meminfo) {
   NOTIMPLEMENTED();  // TODO(fuchsia): https://crbug.com/706592.
   return false;