Fix border-radius clip with composited scrolling.

Border-radius clip with nested overflow divs in
composited-scrolling mode is broken in at least 2
ways. One is that, in the legacy comp-scroll path,
some content would remain invisible when it should
be displayed after scrolling down. The second is
that some contents can bleed outside of the corner
clip.

This CL fixes the first issue, and partially fixes
the second (follow-up bugs have been filed to
finish that).

BUG=365828,363363

Review URL: https://codereview.chromium.org/250273002

git-svn-id: svn://svn.chromium.org/blink/trunk@173970 bbb929c8-8fbe-4397-9dbb-9b2b20218538