Crash in RenderLayerScrollableArea::visibleWidth

This change is a blind crash fix. Based on the stack
trace, it seems that we have a race condition in
RenderLayer initialization:
While creating the RenderLayer, one of the objects
calls back into the associate renderer to get the
RenderLayer. As it hasn't been set-up yet, we crash.

This is blocking the ChromeOS release and I haven't
been able to reproduce the crasher as it requires
a ChromeOS build so no test.

BUG=368062
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/266643006

git-svn-id: svn://svn.chromium.org/blink/trunk@173095 bbb929c8-8fbe-4397-9dbb-9b2b20218538