Provide default implementations for WebLayer functions that will soon be removed.

The WebLayer functions setMaxScrollPosition() and setScrollable() will be
going away after https://codereview.chromium.org/23983047 lands, so we need
a default implementation in the meantime.

BUG=

Review URL: https://codereview.chromium.org/139523002

git-svn-id: svn://svn.chromium.org/blink/trunk@165180 bbb929c8-8fbe-4397-9dbb-9b2b20218538