Remove [LegacyTreatAsPartialInterface] from SVGURIReference

V. easy.

R=haraken
BUG=360435

Review URL: https://codereview.chromium.org/224963017

git-svn-id: svn://svn.chromium.org/blink/trunk@171047 bbb929c8-8fbe-4397-9dbb-9b2b20218538