Implement lifetime observer on RenderWidgetHostViewBase.

content/browser/renderer_host/render_widget_host_view_base_observer.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_BASE_OBSERVER_H_
+#define CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_BASE_OBSERVER_H_
+
+#include "base/macros.h"
+#include "content/common/content_export.h"
+
+namespace content {
+
+class RenderWidgetHostViewBase;
+
+class CONTENT_EXPORT RenderWidgetHostViewBaseObserver {
+ public:
+  // All derived classes must de-register as observers when receiving this
+  // notification.
+  virtual void OnRenderWidgetHostViewBaseDestroyed(
+      RenderWidgetHostViewBase* view);
+
+ protected:
+  RenderWidgetHostViewBaseObserver() = default;
+  virtual ~RenderWidgetHostViewBaseObserver();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostViewBaseObserver);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_RENDERER_HOST_RENDER_WIDGET_HOST_VIEW_BASE_OBSERVER_H_