Define casting helper functions for StyleRuleBase subclasses by DEFINE_TYPE_CASTS.

Some StyleRuleBase subclasses had casting helper functions written
manually. This CL repleaces them with functions auto-generated by
DEFINE_TYPE_CASTS. Also, this add missing helper functions for other
subclasses.

BUG=309516
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/98633002

git-svn-id: svn://svn.chromium.org/blink/trunk@162974 bbb929c8-8fbe-4397-9dbb-9b2b20218538