Always remove registered handlers for deleted node

When a Node is about to get deleted from the Document, we should always
remove any event handlers it has registered to the EventHandlerRegistry
-- regardless of whether the Node has EventTargetData or not. This is
because native code may register nodes as having handlers without
actually inserting any handlers in in the node's event target data.

BUG=393312,393154
TEST=WebViewTest.DeleteElementWithRegisteredHandler

Review URL: https://codereview.chromium.org/392283002

git-svn-id: svn://svn.chromium.org/blink/trunk@178366 bbb929c8-8fbe-4397-9dbb-9b2b20218538