Element should not use confusingAndOftenMisusedAttached

Instead introduce Node::inActiveDocument() and use that. To do this I had
to rename a method of the same name on HTMLMediaElement. All callers in
Element.cpp that were checking attached really just want to know if the
element is in an active document and participating in recalcStyle.

Review URL: https://codereview.chromium.org/26270004

git-svn-id: svn://svn.chromium.org/blink/trunk@160733 bbb929c8-8fbe-4397-9dbb-9b2b20218538