Retire ScopedObserver in /components/page_load_metrics.

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=ryansturm@chromium.org

Bug: 1145565
Change-Id: I96cfee44f213c7a1aaa2f4605b93400a03f5df67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532957
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826463}

components/page_load_metrics/renderer/metrics_render_frame_observer.cc

@@ -93,8 +93,7 @@ class MojoPageTimingSender : public PageTimingSender {
 
 MetricsRenderFrameObserver::MetricsRenderFrameObserver(
     content::RenderFrame* render_frame)
-    : content::RenderFrameObserver(render_frame),
-      scoped_ad_resource_observer_(this) {}
+    : content::RenderFrameObserver(render_frame) {}
 
 MetricsRenderFrameObserver::~MetricsRenderFrameObserver() {
   if (page_timing_metrics_sender_)
@@ -345,12 +344,13 @@ void MetricsRenderFrameObserver::DidCommitProvisionalLoad(
 void MetricsRenderFrameObserver::SetAdResourceTracker(
     subresource_filter::AdResourceTracker* ad_resource_tracker) {
   // Remove all sources and set a new source for the observer.
-  scoped_ad_resource_observer_.RemoveAll();
-  scoped_ad_resource_observer_.Add(ad_resource_tracker);
+  if (scoped_ad_resource_observation_.IsObserving())
+    scoped_ad_resource_observation_.RemoveObservation();
+  scoped_ad_resource_observation_.Observe(ad_resource_tracker);
 }
 
 void MetricsRenderFrameObserver::OnAdResourceTrackerGoingAway() {
-  scoped_ad_resource_observer_.RemoveAll();
+  scoped_ad_resource_observation_.RemoveObservation();
 }
 
 void MetricsRenderFrameObserver::OnAdResourceObserved(int request_id) {

components/page_load_metrics/renderer/metrics_render_frame_observer.h

@@ -9,7 +9,7 @@
 #include <set>
 
 #include "base/macros.h"
-#include "base/scoped_observer.h"
+#include "base/scoped_observation.h"
 #include "components/page_load_metrics/common/page_load_timing.h"
 #include "components/page_load_metrics/renderer/page_resource_data_use.h"
 #include "components/page_load_metrics/renderer/page_timing_metadata_recorder.h"
@@ -146,9 +146,9 @@ class MetricsRenderFrameObserver
   std::unique_ptr<PageResourceDataUse> provisional_frame_resource_data_use_;
   int provisional_frame_resource_id_ = 0;
 
-  ScopedObserver<subresource_filter::AdResourceTracker,
-                 subresource_filter::AdResourceTracker::Observer>
-      scoped_ad_resource_observer_;
+  base::ScopedObservation<subresource_filter::AdResourceTracker,
+                          subresource_filter::AdResourceTracker::Observer>
+      scoped_ad_resource_observation_{this};
 
   // Set containing all request ids that were reported as ads from the renderer.
   std::set<int> ad_request_ids_;