Clear FontFaceCache when CSSFontSelector is detached from Document

CSSFontSelector can outlive Document because it is referenced by
Font too (via FontFallbackList), which can live long time (e.g.
HarfBuzzFontCache holds Font).

This patch allows us to free up font resources referenced by
FontFaceCache when the Document is destroyed. 

BUG=366048
TEST=pass all layout tests

Review URL: https://codereview.chromium.org/399593002

git-svn-id: svn://svn.chromium.org/blink/trunk@178741 bbb929c8-8fbe-4397-9dbb-9b2b20218538