Added missing null check to avoid crash.

Added missing null check for RenderLayerScrollableArea to avoid crash.
This is a fix done by code inspection. That's why we have no test.

Review URL: https://codereview.chromium.org/288143002

git-svn-id: svn://svn.chromium.org/blink/trunk@174176 bbb929c8-8fbe-4397-9dbb-9b2b20218538