Only compute advances in FontHarfBuzz::drawGlyphs for vertical text

Change the drawGlyphs method in FontHarfBuzz.cpp to only compute the
advances in the horizontal text case as the information is not needed
for vertical text. Also move glyphs assignment to where it is used and
change type to GlyphBufferGlyph (typedefed to Glyph).

R=schenney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/61373002

git-svn-id: svn://svn.chromium.org/blink/trunk@161465 bbb929c8-8fbe-4397-9dbb-9b2b20218538