MediaQuery::expressions() should return a reference

Similar change as webkit.org/b/120215

m_expressions is never null so we can just return a reference.

Make ExpressionVector typedef be declared outside the class and
use it in more places.

Review URL: https://chromiumcodereview.appspot.com/23257008

git-svn-id: svn://svn.chromium.org/blink/trunk@156667 bbb929c8-8fbe-4397-9dbb-9b2b20218538