FCMify transparent-scrollbar test

This test is the only test with a transparent CSS-styled scrollbar.  In
compositing mode, we fill parts of the page that haven't been filled by
content with the background color of the page.  In non-compositing mode,
the screen is cleared to white under the transparent scrollbar outside
the viewport.

The same code is used for cases like pinch zoom on short pages (where
the page zoomed all the way out doesn't fill the height of the viewport,
leaving a space to be filled in with something) and it'd be more
complicated and less performant to have to clear underneath scrollbars.

Therefore, just bless this "background color under transparent
scrollbar" as acceptable behavior and rebaseline.

BUG=361729

Review URL: https://codereview.chromium.org/259623004

git-svn-id: svn://svn.chromium.org/blink/trunk@172570 bbb929c8-8fbe-4397-9dbb-9b2b20218538