http://b/issue?id=2501101

Create a concept of blocking messages on destruction similar to that in
WebViewCore.java. This is to prevent what I think is a race condition
caused by an orientation event occuring just before the frame is destroyed,
resulting in the orientation listener being called back just before it
is disabled, but posting its message after the messages have been removed.
This results in the orientation event being delivered to a NULL native frame.