Remove redundant branch and add an UNLIKELY.

This is a subset of previously reviewed CL,
https://codereview.chromium.org/23531010/ that had to be reverted due to
unknown reasons for a performance regression. Hence, relanding in pieces.

R=abarth@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/23536035

git-svn-id: svn://svn.chromium.org/blink/trunk@157397 bbb929c8-8fbe-4397-9dbb-9b2b20218538