Replace ScopedObserver with base::ScopedObservation in /content/browser/devtools.

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=caseq@chromium.org

Change-Id: Ic1010c21616f2349627b9219907376b419d85871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526941
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825403}

content/browser/devtools/protocol/page_handler.cc

@@ -245,14 +245,14 @@ void PageHandler::SetRenderer(int process_host_id,
 
   RenderWidgetHostImpl* widget_host =
       host_ ? host_->GetRenderWidgetHost() : nullptr;
-  if (widget_host && observer_.IsObserving(widget_host))
-    observer_.Remove(widget_host);
+  if (widget_host && observation_.IsObservingSource(widget_host))
+    observation_.RemoveObservation();
 
   host_ = frame_host;
   widget_host = host_ ? host_->GetRenderWidgetHost() : nullptr;
 
   if (widget_host)
-    observer_.Add(widget_host);
+    observation_.Observe(widget_host);
 
   if (video_consumer_ && frame_host) {
     video_consumer_->SetFrameSinkId(
@@ -283,7 +283,8 @@ void PageHandler::RenderWidgetHostVisibilityChanged(
 }
 
 void PageHandler::RenderWidgetHostDestroyed(RenderWidgetHost* widget_host) {
-  observer_.Remove(widget_host);
+  DCHECK(observation_.IsObservingSource(widget_host));
+  observation_.RemoveObservation();
 }
 
 void PageHandler::DidAttachInterstitialPage() {

content/browser/devtools/protocol/page_handler.h

@@ -15,7 +15,7 @@
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
-#include "base/scoped_observer.h"
+#include "base/scoped_observation.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "cc/trees/render_frame_metadata.h"
@@ -237,7 +237,9 @@ class PageHandler : public DevToolsDomainHandler,
   BrowserHandler* browser_handler_;
 
   std::unique_ptr<Page::Frontend> frontend_;
-  ScopedObserver<RenderWidgetHost, RenderWidgetHostObserver> observer_{this};
+
+  base::ScopedObservation<RenderWidgetHost, RenderWidgetHostObserver>
+      observation_{this};
   JavaScriptDialogCallback pending_dialog_;
   base::flat_map<base::UnguessableToken, std::unique_ptr<NavigateCallback>>
       navigate_callbacks_;