Oilpan: Prepare to move RenderSelectionInfoBase to Oilpan heap.

Because RenderSelectionInfoBase subclasses don't add Member<> fields,
RenderSelectionInfoBase::trace is not virtual.

BUG=398342

Review URL: https://codereview.chromium.org/426083005

git-svn-id: svn://svn.chromium.org/blink/trunk@180263 bbb929c8-8fbe-4397-9dbb-9b2b20218538