Hold SkTypeface in a RefPtr in FontPlatformDataHarfBuzz.

This allows removal of refcounting code which is implicit when a smart
pointer is used.

Constructing a RefPtr with -1 is problematic (attempting to call ref()
on it will not work), so the hash table deleted implementation was
changed to use a separate boolean member. This
simplifies the code for this somewhat.

BUG=254509

Review URL: https://chromiumcodereview.appspot.com/19770010

git-svn-id: svn://svn.chromium.org/blink/trunk@154688 bbb929c8-8fbe-4397-9dbb-9b2b20218538