cros: Fix ml::UserActivityManager shutdown crash

SessionManager is released in PostMainMessageLoopRun that
happens before PostDestroyThreads. ml::UserActivityManager
is released with DbusServices PostDestroyThreads. Hence it
crashes on shutdown when trying to un-observe SessionManager.

This CL adds a OnSessionManagerDestroyed to SessionManagerObserver
interface so that ml::UserActivityManager could clean up in time.

Bug: 987010
Change-Id: I05043a3e0b946bfb9e733ad476dacc35e9a9a3b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715401Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680197}

chrome/browser/chromeos/power/ml/user_activity_manager.cc

@@ -306,6 +306,11 @@ void UserActivityManager::HandleSmartDimDecision(
   std::move(callback).Run(dim_deferred_);
 }
 
+void UserActivityManager::OnSessionManagerDestroyed() {
+  session_manager_observer_.RemoveAll();
+  session_manager_ = nullptr;
+}
+
 void UserActivityManager::OnSessionStateChanged() {
   DCHECK(session_manager_);
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);

chrome/browser/chromeos/power/ml/user_activity_manager.h

@@ -117,6 +117,7 @@ class UserActivityManager : public ui::UserActivityObserver,
                               UserActivityEvent::ModelPrediction prediction);
 
   // session_manager::SessionManagerObserver overrides:
+  void OnSessionManagerDestroyed() override;
   void OnSessionStateChanged() override;
 
  private:
@@ -202,7 +203,7 @@ class UserActivityManager : public ui::UserActivityObserver,
                  session_manager::SessionManagerObserver>
       session_manager_observer_;
 
-  session_manager::SessionManager* const session_manager_;
+  session_manager::SessionManager* session_manager_ = nullptr;
 
   mojo::Binding<viz::mojom::VideoDetectorObserver> binding_;
 

components/session_manager/core/session_manager.cc

@@ -24,6 +24,9 @@ SessionManager::SessionManager() {
 SessionManager::~SessionManager() {
   DCHECK_EQ(instance, this);
   SessionManager::SetInstance(nullptr);
+
+  for (auto& observer : observers_)
+    observer.OnSessionManagerDestroyed();
 }
 
 // static

components/session_manager/core/session_manager_observer.h

@@ -16,6 +16,9 @@ namespace session_manager {
 //     http://crbug.com/657149.
 class SessionManagerObserver : public base::CheckedObserver {
  public:
+  // Inovked when session manager is destroyed.
+  virtual void OnSessionManagerDestroyed() {}
+
   // Invoked when session state is changed.
   virtual void OnSessionStateChanged() {}