Don't call AXObjectCache after a node has been removed from a document.
Don't call AXObjectCache after a node has been removed from a document.

AXObjectCache only cares about nodes that are part of a document's tree.
Node::removedFrom() in Element.h is where we tell AXObjectCache to remove
a node from its cache.

However, if an element's attribute is modified after being removed from
a document, Element::attributeChanged calls into AXObjectCache, leading to
a crash. Prevent this by only notifying AXObjectCache when the Element is
part of a document.

BUG=507020

Review URL: https://codereview.chromium.org/1258263002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199685 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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