Introduce WebContentsObserverList to encapsulate common behaviour

Add a dedicated helper struct for holding WebContentsObservers. This
allows us to ensure that iteration over all observers can be detected:

- This can replace existing is_notifying_observer_ WebContentsImpl
member and ensure that it's set for almost all observer iterations.

- This allows us to emit a trace event covering observer iteration.

Escape hatch is kept for now for the three places which want to return
value as soon as matching observer is found.

Also WeakPtr-based logic is added to RenderViewTerminated as some observers
are actually trying to delete WebContents there.

R=nasko@chromium.org

Change-Id: Ic94cf9bbc83b2d9bfe93a3090a4f4af3fd8f60dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343269Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797266}

base/trace_event/builtin_categories.h

@@ -185,6 +185,7 @@
   X(TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler"))                     \
   X(TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"))              \
   X(TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.now"))                 \
+  X(TRACE_DISABLED_BY_DEFAULT("content.verbose"))                        \
   X(TRACE_DISABLED_BY_DEFAULT("cpu_profiler"))                           \
   X(TRACE_DISABLED_BY_DEFAULT("cpu_profiler.debug"))                     \
   X(TRACE_DISABLED_BY_DEFAULT("devtools.screenshot"))                    \

content/browser/renderer_host/render_view_host_impl.cc

@@ -612,6 +612,7 @@ void RenderViewHostImpl::RenderProcessExited(
 
   GetWidget()->RendererExited();
   delegate_->RenderViewTerminated(this, info.status, info.exit_code);
+  // |this| might have been deleted. Do not add code here.
 }
 
 bool RenderViewHostImpl::Send(IPC::Message* msg) {

content/browser/web_contents/web_contents_impl.h

@@ -1395,6 +1395,40 @@ class CONTENT_EXPORT WebContentsImpl : public WebContents,
     WebContentsImpl* focused_web_contents_;
   };
 
+  // Container for WebContentsObservers, which knows when we are iterating over
+  // observer set.
+  class WebContentsObserverList {
+   public:
+    WebContentsObserverList();
+    ~WebContentsObserverList();
+
+    void AddObserver(WebContentsObserver* observer);
+    void RemoveObserver(WebContentsObserver* observer);
+
+    template <class ForEachCallable>
+    void ForEachObserver(const ForEachCallable& callable) {
+      TRACE_EVENT0("content", "Iterating over WebContentsObservers");
+      base::AutoReset<bool> scope(&is_notifying_observers_, true);
+      for (WebContentsObserver& observer : observers_) {
+        TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("content.verbose"),
+                     "Dispatching WebContentsObserver callback");
+        callable(&observer);
+      }
+    }
+
+    bool is_notifying_observers() { return is_notifying_observers_; }
+
+    // Exposed to deal with IPC message handlers which need to stop iteration
+    // early.
+    const base::ObserverList<WebContentsObserver>::Unchecked& observer_list() {
+      return observers_;
+    }
+
+   private:
+    bool is_notifying_observers_ = false;
+    base::ObserverList<WebContentsObserver>::Unchecked observers_;
+  };
+
   // See WebContents::Create for a description of these parameters.
   explicit WebContentsImpl(BrowserContext* browser_context);
 
@@ -1719,7 +1753,7 @@ class CONTENT_EXPORT WebContentsImpl : public WebContents,
   // This MUST be listed above frame_tree_ since at destruction time the
   // latter might cause RenderViewHost's destructor to call us and we might use
   // the observer list then.
-  base::ObserverList<WebContentsObserver>::Unchecked observers_;
+  WebContentsObserverList observers_;
 
   // Associated interface receiver sets attached to this WebContents.
   std::map<std::string, WebContentsReceiverSet*> receiver_sets_;