Use correct HRTF elevation response for negative elevations.

The computed index for negative elevations was off by one. When the
elevation is -45 deg, the selected response corresponded to the entry
for 90 deg, not 315 deg.

To simplify things, we just use a short table to map the index to the
(positive) elevation angle.

No tests for this, but see crbug.com/411191.
BUG=412442

Review URL: https://codereview.chromium.org/565303003

git-svn-id: svn://svn.chromium.org/blink/trunk@181986 bbb929c8-8fbe-4397-9dbb-9b2b20218538