IDL compiler: add SVG exception to keep_alive_for_gc

Remaining: 12 => 1

BUG=239771
TBR=haraken, kouhei
NOTRY=true

Review URL: https://codereview.chromium.org/177213002

git-svn-id: svn://svn.chromium.org/blink/trunk@167670 bbb929c8-8fbe-4397-9dbb-9b2b20218538