IDL: Remove holder lookup in attribute_getter macro

According to haraken's comment on [1], we can always use info.Holder()
instead of calling findInstanceInPrototypeChain().
Removing one FIXME.

[1] https://codereview.chromium.org/140663012

BUG=345503

Review URL: https://codereview.chromium.org/347773003

git-svn-id: svn://svn.chromium.org/blink/trunk@177072 bbb929c8-8fbe-4397-9dbb-9b2b20218538