Disallow script execution after unload event dispatch in frame detach.

Source/core/frame/Frame.cpp

Index: Source/core/frame/Frame.cpp
diff --git a/Source/core/frame/Frame.cpp b/Source/core/frame/Frame.cpp
index 269b438b2d2cda5681aa6bea86f07dc5d81cab71..bf88f1c4b0ed1c0b6ada336d6a89768b969e839c 100644
--- a/Source/core/frame/Frame.cpp
+++ b/Source/core/frame/Frame.cpp
@@ -101,12 +101,7 @@ void Frame::trace(Visitor* visitor)
 
 void Frame::detach()
 {
-    // client() should never be null because that means we somehow re-entered
-    // the frame detach code... but it is sometimes.
-    // FIXME: Understand why this is happening so we can document this insanity.
-    // http://crbug.com/371084 is a probable explanation.
-    if (!client())
-        return;
+    ASSERT(m_client);
     disconnectOwnerElement();
     // After this, we must no longer talk to the client since this clears
     // its owning reference back to our owning LocalFrame.