Unify logic for deciding whether m_graphicsLayer draws content.

In particular, get rid of special-casing for scrolling layers. That code may be
wrong in certain cases, such as paint-into-ancestor and paintsChildren().

BUG=409593

Review URL: https://codereview.chromium.org/536143003

git-svn-id: svn://svn.chromium.org/blink/trunk@181390 bbb929c8-8fbe-4397-9dbb-9b2b20218538