Make Range::nodeWillBeRemoved() to accept removed node

This patch changes Range::nodeWillBeRemoved() to accept remove node by replacing ASSERT to if-statement. Range::nodeWillBeRemoved() might be called with removed node, Node::parentNode() == nullptr, when DOMNodeRemovedFromDocument event handler calls removeChild(), directly or indirectly, for node being removed.

This patch is a part of fixing issue 240594 for reproducing the reported situation.

BUG=240594
TEST=LayoutTests/fast/dom/Range/remove-twice-crash.html
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/23709004

git-svn-id: svn://svn.chromium.org/blink/trunk@157193 bbb929c8-8fbe-4397-9dbb-9b2b20218538