Defer call to updateWidgetPositions() outside of RenderLayerScrollableArea.

updateWidgetPositions() can destroy the render tree, so it should never
be called from inside RenderLayerScrollableArea. Leaving it there allows
for the potential of use-after-free bugs.

BUG=402407
R=vollick@chromium.org

Review URL: https://codereview.chromium.org/490473003

git-svn-id: svn://svn.chromium.org/blink/trunk@180681 bbb929c8-8fbe-4397-9dbb-9b2b20218538