SVGPropertyTearOffs should detachChildren before deleting its value.

TEST=svg/transforms/svg-matrix-tearoff-crash.html
BUG=296276

Review URL: https://codereview.chromium.org/25314003

git-svn-id: svn://svn.chromium.org/blink/trunk@158563 bbb929c8-8fbe-4397-9dbb-9b2b20218538