Introduce RenderProcessHostObserver, use it in its first client.

content/public/browser/render_process_host_observer.h

+// Copyright 2013 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_PUBLIC_BROWSER_RENDER_PROCESS_HOST_OBSERVER_H_
+#define CONTENT_PUBLIC_BROWSER_RENDER_PROCESS_HOST_OBSERVER_H_
+
+#include "content/common/content_export.h"
+
+namespace content {
+
+class RenderProcessHost;
+
+// An observer API implemented by classes which are interested
+// in RenderProcessHost lifecycle events.
+class CONTENT_EXPORT RenderProcessHostObserver {
+ public:
+  // Called when the observed RenderProcessHost itself is destroyed.
+  virtual void RenderProcessHostDestroyed(RenderProcessHost* host) {}
+
+ protected:
+  virtual ~RenderProcessHostObserver() {}
+};
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_BROWSER_RENDER_PROCESS_HOST_OBSERVER_H_