Oilpan: make ViewportAnchor fully stack allocated.

Mark this class with STACK_ALLOCATED() and convert its on-heap references
into Members as a result.

R=ager@chromium.org,haraken@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/395843002

git-svn-id: svn://svn.chromium.org/blink/trunk@178250 bbb929c8-8fbe-4397-9dbb-9b2b20218538