&zwj; and &zwnj; should not be treated as missing glyphs.

This is for fixing the regression I introduced in r153776, which meant to be
a workaround against Windows Vista that sometimes return zero-width glyph for
a missing glyph. However, obviously, truely zero-width characters should not
be regarded as missing glyphs. Most of such characters were already excepted out,
but &zwj; and &zwnj; were not handled.

BUG=262113, 120336

Review URL: https://chromiumcodereview.appspot.com/20019002

git-svn-id: svn://svn.chromium.org/blink/trunk@154727 bbb929c8-8fbe-4397-9dbb-9b2b20218538