We were using a formula that didn't work for some of the clipping
tests.

Bug: angleproject:2601
Change-Id: I60d965c2701656ad590c3b250c2cc777fb432421
Reviewed-on: https://chromium-review.googlesource.com/1194919Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>