Hold SVGFontFaceElement in SVGDocumentExtensions until StyleRecalc is finished.

- added HashSet holding SVGFontFaceElement and cleared after document's style recalc is finished, and

- made widthIterator return default value when SVGFontFaceElement is not in document.

BUG=352178
TEST=fast/dom/remove-svg-font-face-element-crash.xhtml

Review URL: https://codereview.chromium.org/198783006

git-svn-id: svn://svn.chromium.org/blink/trunk@169774 bbb929c8-8fbe-4397-9dbb-9b2b20218538