Make FontListImpl RefCountedThreadSafe
FontListImpl has non-thread-safe ref count, and this CL replaces it
with thread safe ref count.
FontListImpl is held by FontList, and |font_cache_| in ResourceBundle
holds FontLists. |font_cache_| itself is accessed from multiple thread
with a protection of |images_and_fonts_lock_|, but its contents seem
exposed without any protection.
That implies there is a potential data race if a user of ResourceBundle
takes a FontList from GetFontList(), holds a copy of it, and
simultaneously touches |font_cache_| by ReloadLocaleResources on