Require the entire document-tree to be single-origin for SVGImage

SVGImage::currentFrameHasSingleSecurityOrigin only checks the
first occurance of an <svg:image> or and <svg:feImage>, which
meant that if the first such element had a benign reference,
any later elements would be ignored.

Also, since |node| can only ever match one of the tagnames, use a
cascading 'if' where appropriate.

Review URL: https://codereview.chromium.org/163573003

git-svn-id: svn://svn.chromium.org/blink/trunk@167097 bbb929c8-8fbe-4397-9dbb-9b2b20218538