Make scrollbars in pinch viewport invisible until compositor makes them visible.

This will prevent the pinch viewport overlay scrollbars from showing up
in layout tests. Does not change visibility in normal browser usage.

BUG=148816

Review URL: https://codereview.chromium.org/689543004

git-svn-id: svn://svn.chromium.org/blink/trunk@184669 bbb929c8-8fbe-4397-9dbb-9b2b20218538