IDL compiler: Element wrappers for SVGElement

40 => 39 (fixes last .h file)
One more special case.

BUG=239771
TBR=haraken, kouhei
NOTRY=true

Review URL: https://codereview.chromium.org/177013002

git-svn-id: svn://svn.chromium.org/blink/trunk@167664 bbb929c8-8fbe-4397-9dbb-9b2b20218538