PM: Use base::ScopedObservation in WorkerWatcher.

Also move ScopedObservation into the base namespace. It's an
oversight that it wasn't there in the first place.

Change-Id: I854bc0c96af72dff096894cd3b82cd7710f6add3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519271Reviewed-by: Patrick Monette <pmonette@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824123}

base/scoped_observation.h

@@ -9,6 +9,8 @@
 
 #include "base/check_op.h"
 
+namespace base {
+
 // ScopedObservation is used to keep track of a single observation.
 // When ScopedObservation is destroyed, it removes the registered observation,
 // if any. Basic example (as a member variable):
@@ -74,4 +76,6 @@ class ScopedObservation {
   Source* source_ = nullptr;
 };
 
+}  // namespace base
+
 #endif  // BASE_SCOPED_OBSERVATION_H_

components/performance_manager/service_worker_context_adapter.cc

@@ -6,6 +6,7 @@
 
 #include "base/check_op.h"
 #include "base/notreached.h"
+#include "base/scoped_observation.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/render_process_host_observer.h"
 
@@ -37,8 +38,10 @@ class ServiceWorkerContextAdapter::RunningServiceWorker
   // The adapter that owns |this|. Notified when RenderProcessExited() is
   // called.
   ServiceWorkerContextAdapter* const adapter_;
-  // The render process host this instance is observing.
-  content::RenderProcessHost* observing_ = nullptr;
+
+  base::ScopedObservation<content::RenderProcessHost,
+                          content::RenderProcessHostObserver>
+      scoped_observation_{this};
 };
 
 ServiceWorkerContextAdapter::RunningServiceWorker::RunningServiceWorker(
@@ -47,20 +50,19 @@ ServiceWorkerContextAdapter::RunningServiceWorker::RunningServiceWorker(
     : version_id_(version_id), adapter_(adapter) {}
 
 ServiceWorkerContextAdapter::RunningServiceWorker::~RunningServiceWorker() {
-  DCHECK_EQ(observing_, nullptr);
+  DCHECK(!scoped_observation_.IsObserving());
 }
 
 void ServiceWorkerContextAdapter::RunningServiceWorker::Subscribe(
     content::RenderProcessHost* worker_process_host) {
-  DCHECK_EQ(observing_, nullptr);
-  worker_process_host->AddObserver(this);
-  observing_ = worker_process_host;
+  DCHECK(!scoped_observation_.IsObserving());
+  scoped_observation_.Observe(worker_process_host);
 }
 
 void ServiceWorkerContextAdapter::RunningServiceWorker::Unsubscribe() {
-  DCHECK_NE(observing_, nullptr);
-  observing_->RemoveObserver(this);
-  observing_ = nullptr;
+  DCHECK(scoped_observation_.IsObserving());
+
+  scoped_observation_.RemoveObservation();
 }
 
 void ServiceWorkerContextAdapter::RunningServiceWorker::RenderProcessExited(