Revert "Retire ScopedObserver in HungPagesTableModel".

Instead of trying to forward fix, dumbly revert the cause of crashes.
Hopefully, just in time before M89 branch cut.

This revert will fix:
- https://crbug.com/1153966
- https://crbug.com/1153961

This revert part of:
https://chromium-review.googlesource.com/c/chromium/src/+/2548022
```
Retire ScopedObserver in /chrome/browser/ui/views.

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.

This CL was uploaded by git cl split.

R=bsep@chromium.org

Bug: 1145565
```

There was an existing bug. Unexpectedly, HungPagesTableModel::
InitForWebContents() can be called twice. See
https://crbug.com/1165917. For some reasons, this didn't resulted in any
crashes...

...until the reverted patch cames in.
The old base::ScopedObserver was supporting multiple observations. The
new one: base::ScopedObservation doesn't. As a result, if
base::ScopedObservation::Observe is called twice, then RemoveObserver
will be called only once instead of two, causing the issue.

Bug: 1153966,1153961
Change-Id: Iecc8e8e0e9fa387795ce55b4414d1d871cc4c9a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625867Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842940}

chrome/browser/ui/views/hung_renderer_view.cc

@@ -80,8 +80,8 @@ void HungPagesTableModel::InitForWebContents(
   DCHECK(!hang_monitor_restarter.is_null());
 
   DCHECK(!render_widget_host_);
-  DCHECK(!process_observation_.IsObserving());
-  DCHECK(!widget_observation_.IsObserving());
+  DCHECK(!process_observer_.IsObservingSources());
+  DCHECK(!widget_observer_.IsObservingSources());
   DCHECK(tab_observers_.empty());
 
   render_widget_host_ = render_widget_host;
@@ -93,8 +93,8 @@ void HungPagesTableModel::InitForWebContents(
         std::make_unique<WebContentsObserverImpl>(this, hung_contents));
   }
 
-  process_observation_.Observe(render_widget_host_->GetProcess());
-  widget_observation_.Observe(render_widget_host_);
+  process_observer_.Add(render_widget_host_->GetProcess());
+  widget_observer_.Add(render_widget_host_);
 
   // The world is different.
   if (observer_)
@@ -102,8 +102,8 @@ void HungPagesTableModel::InitForWebContents(
 }
 
 void HungPagesTableModel::Reset() {
-  process_observation_.Reset();
-  widget_observation_.Reset();
+  process_observer_.RemoveAll();
+  widget_observer_.RemoveAll();
   tab_observers_.clear();
   render_widget_host_ = nullptr;
 
@@ -158,8 +158,8 @@ void HungPagesTableModel::RenderProcessExited(
 
 void HungPagesTableModel::RenderWidgetHostDestroyed(
     content::RenderWidgetHost* widget_host) {
-  DCHECK(widget_observation_.IsObservingSource(render_widget_host_));
-  widget_observation_.Reset();
+  DCHECK(widget_observer_.IsObserving(render_widget_host_));
+  widget_observer_.Remove(widget_host);
   render_widget_host_ = nullptr;
 
   // Notify the delegate.

chrome/browser/ui/views/hung_renderer_view.h

@@ -10,7 +10,7 @@
 
 #include "base/callback.h"
 #include "base/macros.h"
-#include "base/scoped_observation.h"
+#include "base/scoped_observer.h"
 #include "components/favicon/content/content_favicon_driver.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/render_process_host_observer.h"
@@ -124,13 +124,11 @@ class HungPagesTableModel : public ui::TableModel,
   // some more until the renderer process responds).
   base::RepeatingClosure hang_monitor_restarter_;
 
-  base::ScopedObservation<content::RenderProcessHost,
-                          content::RenderProcessHostObserver>
-      process_observation_{this};
+  ScopedObserver<content::RenderProcessHost, content::RenderProcessHostObserver>
+      process_observer_{this};
 
-  base::ScopedObservation<content::RenderWidgetHost,
-                          content::RenderWidgetHostObserver>
-      widget_observation_{this};
+  ScopedObserver<content::RenderWidgetHost, content::RenderWidgetHostObserver>
+      widget_observer_{this};
 
   DISALLOW_COPY_AND_ASSIGN(HungPagesTableModel);
 };