IDL: Remove UNLIKELY() from constructor

To remove a FIXME comment. I ran Dromaeo and didn't see any regression.
Here is the result.

w/ CL
1st: 1646.81runs/s (Total)
http://dromaeo.com/?id=224905

2nd: 1621.15runs/s (Total)
http://dromaeo.com/?id=224906

w/o CL
1st: 1662.16runs/s (Total)
http://dromaeo.com/?id=224907

2nd: 1644.60runs/s (Total)
http://dromaeo.com/?id=224908

Also, remove an obsolated FIXME comment.

BUG=345503

Review URL: https://codereview.chromium.org/419673003

git-svn-id: svn://svn.chromium.org/blink/trunk@178934 bbb929c8-8fbe-4397-9dbb-9b2b20218538