Crash in RenderLayer::repaintBlockSelection

This is a blind fix as I couldn't get a reproduction from our
crashes. From the traces, it looks like a NULL scrollable area
when accessing scrolledContentOffset but for good measure, I
added the NULL check everywhere.

BUG=302128

Review URL: https://codereview.chromium.org/25729003

git-svn-id: svn://svn.chromium.org/blink/trunk@158965 bbb929c8-8fbe-4397-9dbb-9b2b20218538