Avoid checking adjacent rules when doing a Force recalc or if no children changed

Iterating the children is a waste if we're already going to recalc
everyone like in a Force situation or if we know that no children
needStyleRecalc(). We should skip all the checking in these cases.

I also refactored the loop into checkForChildrenAdjacentRuleChanges()
since recalcChildStyle was getting too long.

Review URL: https://codereview.chromium.org/59553002

git-svn-id: svn://svn.chromium.org/blink/trunk@161364 bbb929c8-8fbe-4397-9dbb-9b2b20218538