oobe UpdateScreen: Make PowerManagerClient observer scoped

Now UpdateScreen observes power changes only when it is shown.

Bug: 1124201
Change-Id: Ic67f2e3c4fc6c6fb528235d74faa2f2929c819b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390220
Commit-Queue: Danila Kuzmin <dkuzmin@google.com>
Reviewed-by: Roman Aleksandrov <raleksandrov@google.com>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803882}

chrome/browser/chromeos/login/screens/update_screen.cc

@@ -120,15 +120,11 @@ UpdateScreen::UpdateScreen(UpdateView* view,
       version_updater_(std::make_unique<VersionUpdater>(this)),
       wait_before_reboot_time_(kWaitBeforeRebootTime),
       tick_clock_(base::DefaultTickClock::GetInstance()) {
-  if (chromeos::features::IsBetterUpdateEnabled())
-    PowerManagerClient::Get()->AddObserver(this);
   if (view_)
     view_->Bind(this);
 }
 
 UpdateScreen::~UpdateScreen() {
-  if (chromeos::features::IsBetterUpdateEnabled())
-    PowerManagerClient::Get()->RemoveObserver(this);
   if (view_)
     view_->Unbind();
 }
@@ -164,8 +160,13 @@ bool UpdateScreen::MaybeSkip(WizardContext* context) {
 }
 
 void UpdateScreen::ShowImpl() {
-  if (chromeos::features::IsBetterUpdateEnabled())
+  if (chromeos::features::IsBetterUpdateEnabled()) {
+    DCHECK(!power_manager_subscription_);
+    power_manager_subscription_ = std::make_unique<
+        ScopedObserver<PowerManagerClient, PowerManagerClient::Observer>>(this);
+    power_manager_subscription_->Add(PowerManagerClient::Get());
     PowerManagerClient::Get()->RequestStatusUpdate();
+  }
 #if !BUILDFLAG(GOOGLE_CHROME_BRANDING)
   if (view_) {
     view_->SetCancelUpdateShortcutEnabled(true);
@@ -181,6 +182,7 @@ void UpdateScreen::ShowImpl() {
 }
 
 void UpdateScreen::HideImpl() {
+  power_manager_subscription_.reset();
   show_timer_.Stop();
   if (view_)
     view_->Hide();

chrome/browser/chromeos/login/screens/update_screen.h

@@ -12,6 +12,7 @@
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
+#include "base/scoped_observer.h"
 #include "base/timer/timer.h"
 #include "chrome/browser/chromeos/login/screens/base_screen.h"
 #include "chrome/browser/chromeos/login/screens/error_screen.h"
@@ -221,6 +222,11 @@ class UpdateScreen : public BaseScreen,
 
   ErrorScreen::ConnectRequestCallbackSubscription connect_request_subscription_;
 
+  // PowerManagerClient::Observer is used only when screen is shown.
+  std::unique_ptr<
+      ScopedObserver<PowerManagerClient, PowerManagerClient::Observer>>
+      power_manager_subscription_;
+
   base::WeakPtrFactory<UpdateScreen> weak_factory_{this};
 
   DISALLOW_COPY_AND_ASSIGN(UpdateScreen);