[SVG] Disallow SVGViewSpec.viewBox tearoff creation after contextElement is destroyed.

BUG=336436
BUG=308818

Review URL: https://codereview.chromium.org/141943004

git-svn-id: svn://svn.chromium.org/blink/trunk@165566 bbb929c8-8fbe-4397-9dbb-9b2b20218538