Adjust both x and y position when drawing glyphs

Change GlyphBuffer::advanceAt to return a FloatSize with both the x and
y advances. Change Font::drawGlyphBuffer to use the y advance, in
addition to the x advance, when adjusting the startPoint for drawing
glyphs.

R=dglazkov@chromium.org
BUG=247103
TEST=fast/text/international/vertical-positioning-with-combining-marks.html

Review URL: https://codereview.chromium.org/180893004

git-svn-id: svn://svn.chromium.org/blink/trunk@168670 bbb929c8-8fbe-4397-9dbb-9b2b20218538