Oilpan: Move SelectorFilter to oilpan's heap

This CL moves SelectorFilter::ParentStackFrame from a private section to a public section to use a WTF_ALLOW_INIT_WITH_MEM_FUNCTIONS macro.

BUG=340522

Review URL: https://codereview.chromium.org/422143005

git-svn-id: svn://svn.chromium.org/blink/trunk@179079 bbb929c8-8fbe-4397-9dbb-9b2b20218538