Auto-generate casting helper functions for CSSRule subclasses.

Introduce DEFINE_CSS_RULE_TYPE_CASTS macro, and use it for all of CSSRule
subclasses.
Replace existing static_cast<CSSFooRule*> with toCSSFooRule().

This CL is helpful to detect bad casts.

BUG=309516
TEST=none; No behavior changes.

Review URL: https://codereview.chromium.org/108923005

git-svn-id: svn://svn.chromium.org/blink/trunk@163508 bbb929c8-8fbe-4397-9dbb-9b2b20218538