Remove FIXME in StyleResolver::styleForElement

The action suggested in the FIXME has the downside of not doing
the right thing when the body has display set to none, as the
test shows. So remove the FIXME and keep the old behavior.

Review URL: https://codereview.chromium.org/468503003

git-svn-id: svn://svn.chromium.org/blink/trunk@180478 bbb929c8-8fbe-4397-9dbb-9b2b20218538