Oilpan: deprecate GCInfo::typeMarker and replace its checks w/ gcInfo ptr compare

BUG=None

Review URL: https://codereview.chromium.org/180533004

git-svn-id: svn://svn.chromium.org/blink/trunk@168616 bbb929c8-8fbe-4397-9dbb-9b2b20218538