SVGGlyphRefElement shouldn't use bloatedCharacters

Another use via AtomicString.

R=pdr@chromium.org
BUG=254708

Review URL: https://codereview.chromium.org/19284008

git-svn-id: svn://svn.chromium.org/blink/trunk@154273 bbb929c8-8fbe-4397-9dbb-9b2b20218538