Share recalcChildStyle between Element and ShadowRoot.

Instead of needing to update both loops any time we change anything we
should just put the Element specific things in Element::recalcStyle and
then put the shared method into ContainerNode. I also removed an old
comment about ShadowRoot::recalcStyle not supporting adjacent selectors
properly (I fixed that bug a long time ago).

There should be no change in behavior, I just moved the code around.

BUG=399816

Review URL: https://codereview.chromium.org/548693002

git-svn-id: svn://svn.chromium.org/blink/trunk@181500 bbb929c8-8fbe-4397-9dbb-9b2b20218538