Make StyleElement robust against tree mutation

It is possible that HTMLStyleElement::removedFrom() is called
before HTMLStyleElement::didNotifySubtreeInsertionsToDocument().

BUG=356653
TEST=append-child-style-crash.html,javascript-url-style-crash.html
R=esprehn@chromium.org, abath@chromium.org

Review URL: https://codereview.chromium.org/221673003

git-svn-id: svn://svn.chromium.org/blink/trunk@170702 bbb929c8-8fbe-4397-9dbb-9b2b20218538