Oilpan: Temporary fix for leaks through mapping from element to attribute node list.

I think we need to bite the bullet and change the semantics of our
weak collections. These leaks are really hard to notice when you are
programming. See crbug.com/381629 which is the bug report I have
filed for tracking that.

R=erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, sigbjornf@opera.com
BUG=381629

Review URL: https://codereview.chromium.org/317203005

git-svn-id: svn://svn.chromium.org/blink/trunk@175670 bbb929c8-8fbe-4397-9dbb-9b2b20218538