Revert of Revert of Issue FrameDestructionObserver::frameDestroyed() notification on detach.

Source/core/frame/LocalFrame.cpp

Index: Source/core/frame/LocalFrame.cpp
diff --git a/Source/core/frame/LocalFrame.cpp b/Source/core/frame/LocalFrame.cpp
index 59c68fb30f1656f1e87a4186e7e690d9dae676e4..a26e93efdc70c55ceac931965c7913172a1ece68 100644
--- a/Source/core/frame/LocalFrame.cpp
+++ b/Source/core/frame/LocalFrame.cpp
@@ -206,15 +206,7 @@
 #if !ENABLE(OILPAN)
     // Oilpan: see setDOMWindow() comment why it is acceptable not to
     // mirror the non-Oilpan call below.
-    //
-    // Also, FrameDestructionObservers that live longer than this
-    // frame object keep weak references to the frame; those will be
-    // automatically cleared by the garbage collector. Hence, explicit
-    // frameDestroyed() notifications aren't needed.
     setDOMWindow(nullptr);
-
-    for (const auto& frameDestructionObserver : m_destructionObservers)
-        frameDestructionObserver->frameDestroyed();
 #endif
 }
 
@@ -291,6 +283,11 @@
     // finalization. Too late to access various heap objects at that
     // stage.
     m_loader.clear();
+
+    // Signal frame destruction here rather than in the destructor.
+    // Main motivation is to avoid being dependent on its exact timing (Oilpan.)
+    for (const auto& frameDestructionObserver : m_destructionObservers)
+        frameDestructionObserver->frameDestroyed();
 }
 
 SecurityContext* LocalFrame::securityContext() const