SVG: Add comment to |loopbreaker| local static var that it is safe

This is a follow up CL to https://codereview.chromium.org/305783004/

BUG=357163
NOTRY=true

Review URL: https://codereview.chromium.org/308793003

git-svn-id: svn://svn.chromium.org/blink/trunk@175093 bbb929c8-8fbe-4397-9dbb-9b2b20218538