Oilpan: Update a comment for the WTF_ANNOTATE_LEAKING_OBJECT_PTR macro in PageMemory

This is a follow-up CL for r173508.

BUG=369429
NOTRY=true

Review URL: https://codereview.chromium.org/278433007

git-svn-id: svn://svn.chromium.org/blink/trunk@173632 bbb929c8-8fbe-4397-9dbb-9b2b20218538