Fix *-baseline computation for {text-,}after-edge/ideographic/baseline

Negative descents were assumed, although they are positive. Fix sign of
'descent' in the {text-,}after-edge and ideographic cases.
Also fix the obvious mistake in the 'baseline' case, where it ought to be
handled in a similar way as 'auto', and not just return the "computed"
alignment-baseline value.

Added an additional case to pre-existing test for alignment-baseline.

TEST=svg/custom/{alignment,dominant}-baseline-modes.svg (pre-existing)
BUG=143990

Review URL: https://codereview.chromium.org/331863014

git-svn-id: svn://svn.chromium.org/blink/trunk@176877 bbb929c8-8fbe-4397-9dbb-9b2b20218538