Unsupported variant selector should return false in harfbuzzGetGlyph

BUG=394081,393896
R=eae,behdad,eseidel

Review URL: https://codereview.chromium.org/399633002

git-svn-id: svn://svn.chromium.org/blink/trunk@178669 bbb929c8-8fbe-4397-9dbb-9b2b20218538