Keep event page alive when there's some Pepper plugin on it.

content/public/browser/web_contents_observer.h

Index: content/public/browser/web_contents_observer.h
diff --git a/content/public/browser/web_contents_observer.h b/content/public/browser/web_contents_observer.h
index b040d64223fe81a143daaedff660bad609e10dd2..bacb7cc768fa39d708ffbce45a1ed1edba3eb68e 100644
--- a/content/public/browser/web_contents_observer.h
+++ b/content/public/browser/web_contents_observer.h
@@ -278,6 +278,11 @@ class CONTENT_EXPORT WebContentsObserver : public IPC::Listener,
   virtual void AppCacheAccessed(const GURL& manifest_url,
                                 bool blocked_by_policy) {}
 
+  // These methods are invoked when a Pepper plugin instance is created/deleted
+  // in the DOM.
+  virtual void PepperInstanceCreated() {}
+  virtual void PepperInstanceDeleted() {}
+
   // Notification that a plugin has crashed.
   // |plugin_pid| is the process ID identifying the plugin process. Note that
   // this ID is supplied by the renderer process, so should not be trusted.