Remove ReaderModeIconView from observer list where needed.

This should either fix the attached bug or at least help with debugging
by DCHECKing when the icon is destroyed without being removed from all
observer lists.

Bug: 1017341
Change-Id: I8a7457f7f82f3dc4936fb60112742af2a05a7d70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006468Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#734057}

chrome/browser/ui/views/reader_mode/reader_mode_icon_view.cc

@@ -23,6 +23,13 @@ ReaderModeIconView::ReaderModeIconView(
                          icon_label_bubble_delegate,
                          page_action_icon_delegate) {}
 
+ReaderModeIconView::~ReaderModeIconView() {
+  content::WebContents* contents = web_contents();
+  if (contents)
+    dom_distiller::RemoveObserver(contents, this);
+  DCHECK(!DistillabilityObserver::IsInObserverList());
+}
+
 void ReaderModeIconView::DidFinishNavigation(
     content::NavigationHandle* navigation_handle) {
   if (GetVisible())
@@ -36,20 +43,29 @@ void ReaderModeIconView::UpdateImpl() {
     return;
   }
 
-  // Notify the icon when navigation to and from a distilled page occurs so that
-  // it can hide the inkdrop.
-  Observe(contents);
-
   if (IsDistilledPage(contents->GetLastCommittedURL())) {
     SetVisible(true);
     SetActive(true);
   } else {
-    dom_distiller::AddObserver(contents, this);
+    // If the currently active web contents has changed since last time, stop
+    // observing the old web contents and start observing the new one.
+    // (WebContentsObserver::web_contents() is not updated until the call to
+    // Observe() below, so it should still contain the old contents.)
+    content::WebContents* old_contents = web_contents();
+    if (old_contents != contents) {
+      if (old_contents)
+        dom_distiller::RemoveObserver(old_contents, this);
+      dom_distiller::AddObserver(contents, this);
+    }
     base::Optional<dom_distiller::DistillabilityResult> distillability =
         dom_distiller::GetLatestResult(contents);
     SetVisible(distillability && distillability.value().is_distillable);
     SetActive(false);
   }
+
+  // Notify the icon when navigation to and from a distilled page occurs so that
+  // it can hide the inkdrop.
+  Observe(contents);
 }
 
 const gfx::VectorIcon& ReaderModeIconView::GetVectorIcon() const {

chrome/browser/ui/views/reader_mode/reader_mode_icon_view.h

@@ -28,7 +28,7 @@ class ReaderModeIconView : public PageActionIconView,
   ReaderModeIconView(CommandUpdater* command_updater,
                      IconLabelBubbleView::Delegate* icon_label_bubble_delegate,
                      PageActionIconView::Delegate* page_action_icon_delegate);
-  ~ReaderModeIconView() override = default;
+  ~ReaderModeIconView() override;
 
  protected:
   // Detect when navigation to the distilled page completes. This is required to

components/dom_distiller/content/browser/distillable_page_utils.h

@@ -38,6 +38,7 @@ std::ostream& operator<<(std::ostream& os, const DistillabilityResult& result);
 class DistillabilityObserver : public base::CheckedObserver {
  public:
   virtual void OnResult(const DistillabilityResult& result) = 0;
+  ~DistillabilityObserver() override = default;
 };
 
 // Add/remove objects to the list of observers to notify when the distillability