This also fixes the C++ side to give the right baseline information.
Previously it was giving the baseline distance for the font, but not
for the actual laid-out text.

I considered also providing a "defaultBaseline" accessor that returns
the distance for the actual dominant baseline, but it turns out right
now we never decide the baseline is ideographic. We always use the
alphabetic baseline. We should probably fix that...

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1200233002.