Make it possible to subclass MediaQueryListListener

...so that C++ code can listen to media query changes.

I can't think of a way to test this directly, so this
will be tested via https://codereview.chromium.org/287163010/ when that lands.

Review URL: https://codereview.chromium.org/361003004

git-svn-id: svn://svn.chromium.org/blink/trunk@177343 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unncessary forward declarations from CSSPropertyParser.h

Some of the forward declarations are not necessary, hence removing them.

BUG: None

Review URL: https://codereview.chromium.org/363643003

git-svn-id: svn://svn.chromium.org/blink/trunk@177344 bbb929c8-8fbe-4397-9dbb-9b2b20218538