Remove [LegacyTreatAsPartialInterface] from SVGFitToViewBox

Easy one

R=haraken
BUG=360435

Review URL: https://codereview.chromium.org/225293005

git-svn-id: svn://svn.chromium.org/blink/trunk@171033 bbb929c8-8fbe-4397-9dbb-9b2b20218538