FreeType automatically selects the 'most Unicode' cmap available,
otherwise the default cmap is NULL. When this happens the fallback
should include the 3,0 symbol cmap, as is outlined in the Apple
TrueType documentation. This cmap should effectively be a Unicode
mapping, but exclusively in the private use area.
BUG=skia:1873,chromium:489452
Review URL: https://codereview.chromium.org/1163833003
This test font contains
☓ U+2613 SALTIRE
⬛ U+2B1B BLACK LARGE SQUARE
⬜ U+2B1C WHITE LARGE SQUARE
All at full EM size (with standard amount above and below the
baseline).
Review URL: https://codereview.chromium.org/1134153002