Fix the overlay-scrollbar ASSERT and register their layers with GraphicsLayer.

The value of the Settings method defaults to false and will only be changed for
layout tests using window.internals and some WebFrameTest tests. It should instead
check the RuntimeEnabledFeatures::overlayScrollbarsEnabled() method.

Also register the scrollbar layer as a contents layer with GraphicsLayer.
These changes were introduced in r157254.

R=wjmaclean
BUG=

Review URL: https://chromiumcodereview.appspot.com/23894006

git-svn-id: svn://svn.chromium.org/blink/trunk@157307 bbb929c8-8fbe-4397-9dbb-9b2b20218538