Disallow adding ContextLifecycleObservers during iteration.

Source/core/frame/FrameDestructionObserver.h

Index: Source/core/frame/FrameDestructionObserver.h
diff --git a/Source/core/frame/FrameDestructionObserver.h b/Source/core/frame/FrameDestructionObserver.h
index 601166c2c57603b4d2209c9b439817d9bd1b73f0..98b1403c4bab26b6a147a57b2bdef9b843d01cd3 100644
--- a/Source/core/frame/FrameDestructionObserver.h
+++ b/Source/core/frame/FrameDestructionObserver.h
@@ -38,7 +38,7 @@ class LocalFrameLifecycleNotifier;
 
 class CORE_EXPORT FrameDestructionObserver : public LifecycleObserver<LocalFrame, FrameDestructionObserver, LocalFrameLifecycleNotifier> {
 public:
-    virtual void willDetachFrameHost();
+    virtual void willDetachFrameHost() { }
     virtual void contextDestroyed()
     {
         setContext(nullptr);
@@ -47,7 +47,10 @@ public:
     LocalFrame* frame() const { return lifecycleContext(); }
 
 protected:
-    explicit FrameDestructionObserver(LocalFrame*);
+    explicit FrameDestructionObserver(LocalFrame* frame)
+        : LifecycleObserver(frame)
+    {
+    }
 };
 
 } // namespace blink