Remove PageVisibilityObserver contextDestroyed() notifications.

third_party/WebKit/Source/platform/LifecycleObserver.h

Index: third_party/WebKit/Source/platform/LifecycleObserver.h
diff --git a/third_party/WebKit/Source/platform/LifecycleObserver.h b/third_party/WebKit/Source/platform/LifecycleObserver.h
index e0d21b0d5c36e8cafe5c9748099d2301a73619eb..a1ac632d3ffca656a2202b19af59dfdd03e9e633 100644
--- a/third_party/WebKit/Source/platform/LifecycleObserver.h
+++ b/third_party/WebKit/Source/platform/LifecycleObserver.h
@@ -39,8 +39,6 @@ class LifecycleObserver : public GarbageCollectedMixin {
  public:
   DEFINE_INLINE_VIRTUAL_TRACE() { visitor->trace(m_lifecycleContext); }
 
-  virtual void contextDestroyed(Context*) {}
-
   Context* lifecycleContext() const { return m_lifecycleContext; }
 
   void clearContext() { setContext(nullptr); }