Retire ScopedObserver in /headless.

ScopedObserver is being deprecated in favor of two new classes:
- base::ScopedObservation for observers that only ever observe
  a single source.
- base::ScopedMultiSourceObservation for observers that do or may
  observe more than a single source.

Bug: 1145565
Change-Id: Icbc05ba48d1768afe3157505279d2d133383b3d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2927660
Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#888073}

headless/lib/browser/headless_focus_client.cc

@@ -9,8 +9,7 @@
 
 namespace headless {
 
-HeadlessFocusClient::HeadlessFocusClient()
-    : focused_window_(nullptr), observer_manager_(this) {}
+HeadlessFocusClient::HeadlessFocusClient() : focused_window_(nullptr) {}
 
 HeadlessFocusClient::~HeadlessFocusClient() = default;
 
@@ -28,12 +27,14 @@ void HeadlessFocusClient::FocusWindow(aura::Window* window) {
   if (window && !window->CanFocus())
     return;
 
-  if (focused_window_)
-    observer_manager_.Remove(focused_window_);
+  if (focused_window_) {
+    DCHECK(observation_manager_.IsObservingSource(focused_window_));
+    observation_manager_.Reset();
+  }
   aura::Window* old_focused_window = focused_window_;
   focused_window_ = window;
   if (focused_window_)
-    observer_manager_.Add(focused_window_);
+    observation_manager_.Observe(focused_window_);
 
   for (aura::client::FocusChangeObserver& observer : focus_observers_)
     observer.OnWindowFocused(focused_window_, old_focused_window);

headless/lib/browser/headless_focus_client.h

@@ -7,7 +7,7 @@
 
 #include "base/macros.h"
 #include "base/observer_list.h"
-#include "base/scoped_observer.h"
+#include "base/scoped_observation.h"
 #include "ui/aura/client/focus_client.h"
 #include "ui/aura/window_observer.h"
 
@@ -31,7 +31,8 @@ class HeadlessFocusClient : public aura::client::FocusClient,
   void OnWindowDestroying(aura::Window* window) override;
 
   aura::Window* focused_window_;
-  ScopedObserver<aura::Window, aura::WindowObserver> observer_manager_;
+  base::ScopedObservation<aura::Window, aura::WindowObserver>
+      observation_manager_{this};
   base::ObserverList<aura::client::FocusChangeObserver>::Unchecked
       focus_observers_;