Remove authorOnly parameter in DOMWindow::getMatchedCSSRules method as it is always called with the default value.

This was probably forgotten in https://src.chromium.org/viewvc/blink?revision=70335&view=revision when user agent style
sheets were made unavailable through CSSOM.
It is confusing since it makes one (incorrectly) assume that user agent style sheets are sometimes returned through this API.

R=ojan, eseidel

Review URL: https://codereview.chromium.org/99963008

git-svn-id: svn://svn.chromium.org/blink/trunk@164222 bbb929c8-8fbe-4397-9dbb-9b2b20218538