Remove TreatNullAs=NullString for CSS*Rule

http://dev.w3.org/csswg/cssom/#csscharsetrule
http://dev.w3.org/csswg/css-animations/#csskeyframesrule
http://dev.w3.org/csswg/cssom/#csspagerule
http://dev.w3.org/csswg/cssom/#cssrule
http://dev.w3.org/csswg/cssom/#cssstylerule

The tests for the updated properties pass in Firefox Nightly, IE11
Release Preview and Opera 12.16, with these exceptions:

Firefox reflects CSSCharsetRule.encoding, CSSKeyframesRule.name, and
CSSPageRule.selectorText as null, and ignores setting
CSSStyleRule.selectorText.

IE doesn't support CSSCharsetRule, throws an exception when setting
CSSPageRule.selectorText and ignores setting
CSSStyleRule.selectorText.

Opera throws an exception when setting CSSRule.cssText, ignores
setting CSSCharsetRule.encoding and throws an exception when setting
CSSPageRule.selectorText.

This is not very impressive, but since there is no attribute for which
all the other browsers agree, just going with the spec seems sound.

BUG=310298

Review URL: https://codereview.chromium.org/107313004

git-svn-id: svn://svn.chromium.org/blink/trunk@163424 bbb929c8-8fbe-4397-9dbb-9b2b20218538