Oilpan: tidy up reference to the Document's "CSS target".

Keep a traced Member reference to the Element that's recorded as
the Document's CSS target. When that Element is removed from the
tree, the Member will be explicitly cleared.

R=
BUG=357163

Review URL: https://codereview.chromium.org/315333002

git-svn-id: svn://svn.chromium.org/blink/trunk@175609 bbb929c8-8fbe-4397-9dbb-9b2b20218538