Oilpan: add missing tracing of Attr objects.

The Attr's element needs to be accounted for when tracing.

R=haraken@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/293113008

git-svn-id: svn://svn.chromium.org/blink/trunk@174720 bbb929c8-8fbe-4397-9dbb-9b2b20218538