Restore scroll position when going back to failures view

This patch simply marks <core-animated-pages> children with "overflow: auto",
which besides retaining the scroll position on transitions also makes the
transition look a bit better (as the top bar is always present).

BUG=404033
NOTRY=true

Review URL: https://codereview.chromium.org/511713002

git-svn-id: svn://svn.chromium.org/blink/trunk@180992 bbb929c8-8fbe-4397-9dbb-9b2b20218538