Initialize SystemDataProvider::memory_usage_timer_

Currently uninitialized and would cause a segfault if any clients
attempted to observe memory usage.

Bug: 1128204
Change-Id: I3bb584aa4d69f142dea93372c3322be27b528bb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518116Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Auto-Submit: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823729}

chromeos/components/diagnostics_ui/backend/system_data_provider.cc

@@ -145,6 +145,7 @@ void PopulateMemoryUsage(const healthd::MemoryInfo& memory_info,
 SystemDataProvider::SystemDataProvider() {
   battery_charge_status_timer_ = std::make_unique<base::RepeatingTimer>();
   battery_health_timer_ = std::make_unique<base::RepeatingTimer>();
+  memory_usage_timer_ = std::make_unique<base::RepeatingTimer>();
   PowerManagerClient::Get()->AddObserver(this);
 }