StyleEngine should clear FontSelector before dying

CSSFontSelector can outlive Document and StyleEngine, so StyleEngine
should clean-up the FontSelector on destruction. Otherwise, FontSelector
can have a stale Document pointer.

This is a regression introduced by
https://src.chromium.org/viewvc/blink?view=rev&revision=163369.

BUG=336921

Review URL: https://codereview.chromium.org/146593002

git-svn-id: svn://svn.chromium.org/blink/trunk@165824 bbb929c8-8fbe-4397-9dbb-9b2b20218538