IDL: Remove a FIXME comment for using local variables

Using local variable for isolate in attribute_getter will add
complexity and won't pay for it.

BUG=345503

Review URL: https://codereview.chromium.org/359703002

git-svn-id: svn://svn.chromium.org/blink/trunk@177077 bbb929c8-8fbe-4397-9dbb-9b2b20218538