Use IDL argument default values for Text()/Comment() constructors

The 'data' argument should default to "", not null.

BUG=258153

Review URL: https://codereview.chromium.org/346213004

git-svn-id: svn://svn.chromium.org/blink/trunk@177016 bbb929c8-8fbe-4397-9dbb-9b2b20218538