arc: Fix crash in arc_process_task.cc during browser shutdown

Since TaskManagerImpl is destructed after ~ArcServiceManager is called,
we need to check the return value from the getter.

BUG=680301
TEST=1) try, 2) sign in, wait for ARC boot, press Search+ESC,
 press Ctrl+Shift+QQ to sign out, confirm the browser does not crash

Review-Url: https://codereview.chromium.org/2625823005
Cr-Commit-Position: refs/heads/master@{#443304}

chrome/browser/task_manager/providers/arc/arc_process_task.cc

@@ -113,10 +113,14 @@ void ArcProcessTask::StartIconLoading() {
 }
 
 ArcProcessTask::~ArcProcessTask() {
-  arc::ArcServiceManager::Get()
-      ->arc_bridge_service()
-      ->intent_helper()
-      ->RemoveObserver(this);
+  auto* service_manager = arc::ArcServiceManager::Get();
+  // This destructor can also be called when TaskManagerImpl is destructed.
+  // Since TaskManagerImpl is a LAZY_INSTANCE, arc::ArcServiceManager may have
+  // already been destructed. In that case, arc_bridge_service() has also been
+  // destructed, and it is safe to just return.
+  if (!service_manager)
+    return;
+  service_manager->arc_bridge_service()->intent_helper()->RemoveObserver(this);
 }
 
 Task::Type ArcProcessTask::GetType() const {