IDL: Partial revert of r178934

Some folks raised concerns about removing UNLIKELY() from constructor.
We should revert the change.

TBR=haraken@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/419523004

git-svn-id: svn://svn.chromium.org/blink/trunk@178937 bbb929c8-8fbe-4397-9dbb-9b2b20218538