Rename childrenAffectedBy methods and constants.

For :hover, :active, :focus, and :drag, childrenAffectedBy really means
that descendants, or siblings, or elements in the siblings' subtrees may be
affected. Methods and constants are renamed to reflect that.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/308683007

git-svn-id: svn://svn.chromium.org/blink/trunk@175232 bbb929c8-8fbe-4397-9dbb-9b2b20218538