Check null for WebViewCore's nativeClass before calling

Check null for WebViewCore's nativeClass before calling
back to Java. This may happen if WebKit holds some reference
of the Document even when our Java side has been destroyed.
So when Document finally is deleted, the call back to Java
will have null nativeClass.

In the long run, we may need to add the check for all the
callback to Java.

Fix http://b/issue?id=2148023