IDL compiler: Regularize SVGAnimated* wrapper code

This CL changes the special-case generated code for SVGAnimated*
so it's identical to code generated for [SetWrapperReferenceTo],
specifically [SetWrapperReferenceTo(SVGElement contextElement)].

Followup CL will adds this extended attribute to the IDL files and remove
the special case, but have no code changes itself.

BUG=239771
TBR=haraken, kouhei

Review URL: https://codereview.chromium.org/176953002

git-svn-id: svn://svn.chromium.org/blink/trunk@167718 bbb929c8-8fbe-4397-9dbb-9b2b20218538