Ensure that Slerp returns a non-inf for opposing vectors.

In the case where the given vectors are opposite of each other, we
get a 0 denominator which causes inf/nan values which later down
the pipeline can result in CHECKs since the layer transform would have
nans as well.

R=vollick
BUG=506543

Review URL: https://codereview.chromium.org/1217773011

Cr-Commit-Position: refs/heads/master@{#337881}
2 files changed